Oxygen-containing phosphor powders, methods for making phosphor powders and devices incorporating same

ABSTRACT

Phosphor powders and a method for making phosphor powders. The powders are oxygen-containing, such as metal oxides, silicates, borates or titanates and have a small particle size, narrow particle size distribution and are substantially spherical. The method of the invention advantageously permits the continuous production of such powders. The invention also relates to improved devices, such as display devices, incorporating the phosphor powders.

This application is a divisional application of U.S. patent applicationSer. No. 09/757,391 filed on Jan. 8, 2001, now U.S. Pat. No. 6,555,022,which is a divisional application of U.S. patent application Ser. No.09/028,603, filed on Feb. 24, 1998, now U.S. Pat. No. 6,180,029, whichclaims, priority from U.S. Provisional Patent Application Nos.60/038,262 and 60/039,450, both filed on Feb. 24, 1997. Each of theseapplications is incorporated herein by reference in its entirety.

This invention was made with Government support under contractsN00014-95-C-0278 and N00014-96-C0395 awarded by the Office of NavalResearch. The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to oxygen-containing phosphor powders,methods for producing such powders and devices incorporating same. Inparticular, the present invention is directed to oxygen-containingphosphor powders having small average particle size, a narrow particlesize distribution, high crystallinity and spherical morphology. Thepresent invention also relates to a method for continuously producingsuch oxygen-containing powders and to devices that incorporate suchpowders.

2. Description of Related Art

Phosphors are compounds that are capable of emitting useful quantitiesof radiation in the visible and/or ultraviolet spectrums upon excitationof the material by an external energy source. Due to this property,phosphor compounds have long been utilized in cathode ray tube (CRT)screens for televisions and similar devices. Typically, inorganicphosphor compounds include a host material doped with a small amount ofan activator ion.

More recently, phosphor powders have been utilized in many advanceddisplay devices that provide illuminated text, graphics or video output,including flat panel display devices such as liquid crystal displays,plasma displays, thick film and thin film electroluminescent displaysand field emission displays.

Liquid crystal displays (LCD's) use a low power electric field to modifya light path and are commonly used in wristwatches, pocket televisions,gas pumps, pagers and the like. Plasma displays utilize a gas trappedbetween transparent layers that emits ultraviolet light when excited byan electric field. The ultraviolet light stimulates phosphors on thescreen to emit visible light. Plasma displays are particularly usefulfor larger displays, such as greater than about 20 diagonal inches. Thinfilm and thick film electroluminescent displays (TFEL's) utilize a filmof phosphorescent material trapped between glass plates and electrodesto emit light in an electric field. Such displays are typically used incommercial transportation vehicles, factory floors and emergency rooms.Field emission displays (FED's) are similar in principle to CRT's,wherein electrons emitted from a tip excite phosphors, which then emitlight of different color.

Phosphor powders are also utilized in electroluminescent lamps (EL's),which include phosphor powder deposited on a polymer substrate whichemits light when an electric field is applied.

There are a number of requirements for phosphor powders, which can varydependent upon the specific application of the powder. Generally,phosphor powders should have one or more of the following properties:high purity; high crystallinity; small particle size; narrow particlesize distribution; spherical morphology; controlled surface chemistry;homogenous distribution of the activator ion; good dispersibility; andlow porosity. The proper combination of the foregoing properties willresult in a phosphor powder with high luminescent intensity and longlifetime that can be used in many applications. It is also advantageousfor many applications to provide phosphor powders that are surfacepassivated or coated, such as with a thin, uniform dielectric orsemiconducting coating.

Numerous methods have been proposed for producing oxygen-containingphosphor particles. One such method is referred to as the solid-statemethod. In this process, the phosphor precursor materials are mixed inthe solid state and are heated so that the precursors react and form apowder of the phosphor material. For example, U.S. Pat. No. 4,925,703 byKasenga et al. discloses a method for the production of a manganeseactivated zinc silicate phosphor. The method includes a step of driedblending a mixture of starting components such as zinc oxide, silicicacid and manganese carbonate and firing the blended mixture at about1250° C. The resulting phosphor is broken up or crushed into smallerparticles. Solid-state routes, and many other production methods,utilize such a grinding step to reduce the particle size of the powders.The mechanical grinding damages the surface of the phosphor, formingdead layers which inhibit the brightness of the phosphor powders.

Phosphor powders have also been made by liquid precipitation. In thesemethods, a solution which includes phosphor particle precursors ischemically treated to precipitate phosphor particles or phosphorparticle precursors. These particles are typically calcined at anelevated temperature to produce the phosphor compound. The particlesmust often be further crushed, as is the case with solid-state methods.In yet another method, phosphor particle precursors or phosphorparticles are dispersed in a solution which is then spray dried toevaporate the liquid. The phosphor particles are thereafter sintered inthe solid state at an elevated temperature to crystallize the powder andform a phosphor. For example, U.S. Pat. No. 4,948,527 by Ritsko et al.discloses a process for producing Y₂O₃:Eu phosphors by dispersingyttrium oxide in a europium citrate solution to form a slurry which isthen spray dried. Spray dried powder was then converted to an oxide byfiring at about 1000° C. for two hours and then at 1600° C. for aboutfour hours. The fired powder was then lightly crushed and cleaned torecover useful phosphor particles.

Despite the foregoing, there remains a need for oxygen-containingphosphor powders with high luminescent intensity that include particleshaving a substantially spherical morphology, narrow particle sizedistribution, a high degree of crystallinity and good homogeneity. Thepowder should have good dispersibility and the ability to be fabricatedinto thin layers having uniform thickness, resulting in a device withhigh brightness.

SUMMARY OF THE INVENTION

The present invention provides improved oxygen-containing phosphorpowder batches having a small particle size, narrow particle sizedistribution, spherical morphology and good crystallinity. The presentinvention also provides methods for forming such oxygen-containingphosphor powder batches and devices incorporating such powder batches.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process block diagram showing one embodiment of the processof the present invention.

FIG. 2 is a side view of a furnace and showing one embodiment of thepresent invention for sealing the end of a furnace tube.

FIG. 3 is a view of the side of an end cap that faces away from thefurnace shown in FIG. 2.

FIG. 4 is a view of the side of an end cap that faces toward the furnaceshown in FIG. 2.

FIG. 5 is a side view in cross section of one embodiment of aerosolgenerator of the present invention.

FIG. 6 is a top view of a transducer mounting plate showing a 49transducer array for use in an aerosol generator of the presentinvention.

FIG. 7 is a top view of a transducer mounting plate for a 400 transducerarray for use in an ultrasonic generator of the present invention.

FIG. 8 is a side view, of the transducer mounting plate shown in FIG. 7.

FIG. 9 is a partial side view showing the profile of a single transducermounting receptacle of the transducer mounting plate shown in FIG. 7.

FIG. 10 is a partial side view in cross-section showing an alternativeembodiment for mounting an ultrasonic transducer.

FIG. 11 is a top view of a bottom retaining plate for retaining aseparator for use in an aerosol generator of the present invention.

FIG. 12 is a top view of a liquid feed box having a bottom retainingplate to assist in retaining a separator for use in an aerosol generatorof the present invention.

FIG. 13 is a side view of the liquid feed box shown in FIG. 8.

FIG. 14 is a side view of a gas tube for delivering gas within anaerosol generator of the present invention.

FIG. 15 shows a partial top view of gas tubes positioned in a liquidfeed box for distributing gas relative to ultrasonic transducerpositions for use in an aerosol generator of the present invention.

FIG. 16 shows one embodiment for a gas distribution configuration forthe aerosol generator of the present invention.

FIG. 17 shows another embodiment for a gas distribution configurationfor the aerosol generator of the present invention,

FIG. 18 is a top view of one embodiment of a gas distribution plate/gastube assembly of the aerosol generator of the present invention.

FIG. 19 is a side view of one embodiment of the gas distributionplate/gas tube assembly shown in FIG. 18.

FIG. 20 shows one embodiment for orienting a transducer in the aerosolgenerator of the present invention.

FIG. 21 is a top view of a gas manifold for distributing gas within anaerosol generator of the present invention.

FIG. 22 is a side view of the gas manifold shown in FIG. 21.

FIG. 23 is a top view of a generator lid of a hood design for use in anaerosol generator of the present invention.

FIG. 24 is a side view of the generator lid shown in FIG. 23.

FIG. 25 is a process block diagram of one embodiment in the presentinvention including an aerosol concentrator.

FIG. 26 is a top view in cross section of a virtual impactor that may beused for concentrating an aerosol according to the present invention.

FIG. 27 is a front view of an upstream plate assembly of the virtualimpactor shown in FIG. 26.

FIG. 28 is a top view of the upstream plate assembly shown in FIG. 27.

FIG. 29 is a side view of the upstream plate assembly shown in FIG. 27.

FIG. 30 is a front view of a downstream plate assembly of the virtualimpactor shown in FIG. 26.

FIG. 31 is a top view of the downstream plate assembly shown in FIG. 30.

FIG. 32 is a side view of the downstream plate assembly shown in FIG.30.

FIG. 33 is a process block diagram of one embodiment of the process ofthe present invention including a droplet classifier.

FIG. 34 is a top view in cross section of an impactor of the presentinvention for use in classifying an aerosol.

FIG. 35 is a front view of a flow control plate of the impactor shown inFIG. 34.

FIG. 36 is a front view of a mounting plate of the impactor shown inFIG. 34.

FIG. 37 is a front view of an impactor plate assembly of the impactorshown in FIG. 34.

FIG. 38 is a side view of the impactor plate assembly shown in FIG. 37.

FIG. 39 shows a side view in cross section of a virtual impactor incombination with an impactor of the present invention for concentratingand classifying droplets in an aerosol.

FIG. 40 is a process block diagram of one embodiment of the presentinvention including a particle cooler.

FIG. 41 is a top view of a gas quench cooler of the present invention.

FIG. 42 is an end view of the gas quench cooler shown in FIG. 41.

FIG. 43 is a side view of a perforated conduit of the quench coolershown in FIG. 41.

FIG. 44 is a side view showing one embodiment of a gas quench cooler ofthe present invention connected with a cyclone.

FIG. 45 is a process block diagram of one embodiment of the presentinvention including a particle coater.

FIG. 46 is a block diagram of one embodiment of the present inventionincluding a particle modifier.

FIG. 47 shows cross sections of various particle morphologies of somecomposite particles manufacturable according to the present invention.

FIG. 48 shows a side view of one embodiment of apparatus of the presentinvention including an aerosol generator, an aerosol concentrator, adroplet classifier, a furnace, a particle cooler, and a particlecollector.

FIG. 49 is a block diagram of one embodiment of the process of thepresent invention including the addition of a dry gas between theaerosol generator and the furnace.

FIG. 50 illustrates a schematic view of a CRT device according to anembodiment of the present invention.

FIG. 51 illustrates a schematic representation of pixels on a viewingscreen of a CRT device according to an embodiment of the presentinvention.

FIG. 52 schematically illustrates a plasma display panel according to anembodiment of the present invention.

FIG. 53 schematically illustrates a field emission display according toan embodiment of the present invention.

FIG. 54 illustrates pixel regions on a display device according to theprior art.

FIG. 55 illustrates pixel regions on a display device according to anembodiment of the present invention.

FIG. 56 schematically illustrates a cross-section of anelectroluminescent display device according to an embodiment of thepresent invention.

FIG. 57 schematically illustrates an exploded view of anelectroluminescent display device according to an embodiment of thepresent invention.

FIG. 58 illustrates an electroluminescent lamp according to anembodiment of the present invention.

FIG. 59 illustrates an SEM photomicrograph of an oxygen-containingphosphor powder according to the present invention.

FIG. 60 illustrates an SEM photomicrograph of an oxygen-containingphosphor powder according to the present invention.

DESCRIPTION OF THE INVENTION

The present invention is generally directed to oxygen-containingphosphor powders and methods for producing the powders, as well asdevices which incorporate the powders. As used herein, oxygen-containingphosphor powders, particles and compounds are those which incorporate ahost material that is an oxygen-containing compound, including metaloxides, silicates, borates or aluminates. Specific examples of suchoxygen-containing phosphor compounds are detailed hereinbelow.

In one aspect, the present invention provides a method for preparing aparticulate product. A feed of liquid-containing, flowable medium,including at least one precursor for the desired particulate product, isconverted to aerosol form, with droplets of the medium being dispersedin and suspended by a carrier gas. Liquid from the droplets in theaerosol is then removed to permit formation in a dispersed state of thedesired particles. Typically, the feed precursor is pyrolyzed in afurnace to make the particles. In one embodiment, the particles aresubjected, while still in a dispersed state, to compositional orstructural modification, if desired. Compositional modification mayinclude, for example, coating the particles. Structural modification mayinclude, for example, crystallization, recrystallization ormorphological alteration of the particles. The term powder is often usedherein to refer to the particulate product of the present invention. Theuse of the term powder does not indicate, however, that the particulateproduct must be dry or in any particular environment. Although theparticulate product is typically manufactured in a dry state, theparticulate product may, after manufacture, be placed in a wetenvironment, such as in a paste or slurry.

The process of the present invention is particularly well suited for theproduction of particulate products of finely divided particles having asmall weight average size. In addition to making particles within adesired range of weight average particle size, with the presentinvention the particles may be produced with a desirably narrow sizedistribution, thereby providing size uniformity that is desired for manyapplications.

In addition to control over particle size and size distribution, themethod of the present invention provides significant flexibility forproducing particles of varying composition, crystallinity andmorphology. For example, the present invention may be used to producehomogeneous particles involving only a single phase or multi-phaseparticles including multiple phases. In the case of multi-phaseparticles, the phases may be present in a variety of morphologies. Forexample, one phase may be uniformly dispersed throughout a matrix ofanother phase. Alternatively, one phase may form an interior core whileanother phase forms a coating that surrounds the core. Othermorphologies are also possible, as discussed more fully below.

Referring now to FIG. 1, one embodiment of the process of the presentinvention is described. A liquid feed 102, including at least oneprecursor for the desired particles, and a carrier gas 104 are fed to anaerosol generator 106 where an aerosol 108 is produced. The aerosol 108is then fed to a furnace 110 where liquid in the aerosol 108 is removedto produce particles 112 that are dispersed in and suspended by gasexiting the furnace 110. The particles 112 are then collected in aparticle collector 114 to produce a particulate product 116.

As used herein, the liquid feed 102 is a feed that includes one or moreflowable liquids as the major constituent(s), such that the feed is aflowable medium. The liquid feed 102 need not comprise only liquidconstituents. The liquid feed 102 may comprise only constituents in oneor more liquid phase, or it may also include particulate materialsuspended in a liquid phase. The liquid feed 102 must, however, becapable of being atomized to form droplets of sufficiently small sizefor preparation of the aerosol 108. Therefore, if the liquid feed 102includes suspended particles, those particles should be relatively smallin relation to the size of droplets in the aerosol 108. Such suspendedparticles should typically be smaller than about 1 μm in size,preferably smaller than about 0.5 μm in size, and more preferablysmaller than about 0.3 μm in size and most preferably smaller than about0.1 μm in size. Most preferably, the suspended particles should be ableto form a colloid. The suspended particles could be finely dividedparticles, or could be agglomerate masses comprised of agglomeratedsmaller primary particles. For example, 0.5 μm particles could beagglomerates of nanometer-sized primary particles. When the liquid feed102 includes suspended particles, the particles typically comprise nogreater than about 25 to 50 weight percent of the liquid feed.

As noted, the liquid feed 102 includes at least one precursor forpreparation of the particles 112. The precursor may be a substance ineither a liquid or solid phase of the liquid feed 102. Frequently, theprecursor will be a material, such as a salt, dissolved in a liquidsolvent of the liquid feed 102. The precursor may undergo one or morechemical reactions in the furnace 110 to assist in production of theparticles 112. Alternatively, the precursor material may contribute toformation of the particles 112 without undergoing chemical reaction.This could be the case, for example, when the liquid feed 102 includes,as a precursor material, suspended particles that are not chemicallymodified in the furnace 110. In any event, the particles 112 comprise atleast one component originally contributed by the precursor.

The liquid feed 102 may include multiple precursor materials, which maybe present together in a single phase or separately in multiple phases.For example, the liquid feed 102 may include multiple precursors insolution in a single liquid vehicle. Alternatively, one precursormaterial could be in a solid particulate phase and a second precursormaterial could be in a liquid phase. Also, one precursor material couldbe in one liquid phase and a second precursor material could be in asecond liquid phase, such as could be the case when the liquid feed 102comprises an emulsion. Different components contributed by differentprecursors may be present in the particles together in a single materialphase, or the different components may be present in different materialphases when the particles 112 are composites of multiple phases.Specific examples of preferred precursor materials are discussed morefully below.

The carrier gas 104 may comprise any gaseous medium in which dropletsproduced from the liquid feed 102 may be dispersed in aerosol form.Also, the carrier gas 104 may be inert, in that the carrier gas 104 doesnot participate in formation of the particles 112. Alternatively, thecarrier gas may have one or more active component(s) that contribute toformation of the particles 112. In that regard, the carrier gas mayinclude one or more reactive components that react in the furnace 110 tocontribute to formation of the particles 112. Preferred carrier gascompositions are discussed more fully below.

The aerosol generator 106 atomizes the liquid feed 102 to form dropletsin a manner to permit the carrier gas 104 to sweep the droplets away toform the aerosol 108. The droplets comprise liquid from the liquid feed102. The droplets may, however, also include nonliquid material, such asone or more small particles held in the droplet by the liquid. Forexample, when the particles 112 are composite, or multi-phase,particles, one phase of the composite may be provided in the liquid feed102 in the form of suspended precursor particles and a second phase ofthe composite may be produced in the furnace 110 from one or moreprecursors in the liquid phase of the liquid feed 102. Furthermore theprecursor particles could be included in the liquid feed 102, andtherefore also in droplets of the aerosol 108, for the purpose only ofdispersing the particles for subsequent compositional or structuralmodification during or after processing in the furnace 110.

An important aspect of the present invention is generation of theaerosol 108 with droplets of a small average size, narrow sizedistribution. In this manner, the particles 112 may be produced at adesired small size with a narrow size distribution, which areadvantageous for many applications.

The aerosol generator 106 is capable of producing the aerosol 108 suchthat it includes droplets having a weight average size in a range havinga lower limit of about 1 μm and preferably about 2 μm; and an upperlimit of about 10 μm; preferably about 7 μm, more preferably about 5 μmand most preferably about 4 μm. A weight average droplet size in a rangeof from about 2 μm to about 4 μm is more preferred for mostapplications, with a weight average droplet size of about 3 μm beingparticularly preferred for some applications. The aerosol generator isalso capable of producing the aerosol 108 such that it includes dropletsin a narrow size distribution. Preferably, the droplets in the aerosolare such that at least about 70 percent (more preferably at least about80 weight percent and most preferably at least about 85 weight percent)of the droplets are smaller than about 10 μm and more preferably atleast about 70 weight percent (more preferably at least about 80 weightpercent and most preferably at least about 85 weight percent) aresmaller than about 5 μm. Furthermore, preferably no greater than about30 weight percent, more preferably no greater than about 25 weightpercent and most preferably no greater than about 20 weight percent, ofthe droplets in the aerosol 108 are larger than about twice the weightaverage droplet size.

Another important aspect of the present invention is that the aerosol108 may be generated without consuming excessive amounts of the carriergas 104. The aerosol generator 106 is capable of producing the aerosol108 such that it has a high loading, or high concentration, of theliquid feed 102 in droplet form in that regard, the aerosol 108preferably includes greater than about 1×10⁶ droplets per cubiccentimeter of the aerosol 108, more preferably greater than about 5×10⁶droplets per cubic centimeter, still more preferably greater than about1×10⁷ droplets per cubic centimeter, and most preferably greater thanabout 5×10⁷ droplets per cubic centimeter. That the aerosol generator106 can produce such a heavily loaded aerosol 108 is particularlysurprising considering the high quality of the aerosol 108 with respectto small average droplet size and narrow droplet size distribution.Typically, droplet loading in the aerosol is such that the volumetricratio of liquid feed 102 to carrier gas 104 in the aerosol 108 is largerthan about 0.04 milliliters of liquid feed 102 per liter of carrier gas104 in the aerosol 108, preferably larger than about 0.083 millilitersof liquid feed 102 per liter of carrier gas 104 in the aerosol 108, morepreferably larger than about 0.167 milliliters of liquid feed 102 perliter of carrier gas 104, still more preferably larger than about 0.25milliliters of liquid feed 102 per liter of carrier gas 104, and mostpreferably larger than about 0.333 milliliters of liquid feed 102 perliter of carrier gas 104.

This capability of the aerosol generator 106 to produce a heavily loadedaerosol 108 is even more surprising given the high droplet output rateof which the aerosol generator 106 is capable, as discussed more fullybelow. It will be appreciated that the concentration of liquid feed 102in the aerosol 108 will depend upon the specific components andattributes of the liquid feed 102 and, particularly, the size of thedroplets in the aerosol 108. For example, when the average droplet sizeis from about 2 μm to about 4 μm, the droplet loading is preferablylarger than about 0.15 milliliters of aerosol feed 102 per liter ofcarrier gas 104, more preferably larger than about 0.2 milliliters ofliquid feed 102 per liter of carrier gas 104, even more preferablylarger than about 0.2 milliliters of liquid feed 102 per liter ofcarrier gas 104, and most preferably larger than about 0.3 millilitersof liquid feed 102 per liter of carrier gas 104. When reference is madeherein to liters of carrier gas 104, it refers to the volume that thecarrier gas 104 would occupy under conditions of standard temperatureand pressure.

The furnace 110 may be any suitable device for heating the aerosol 108to evaporate liquid from the droplets of the aerosol 108 and therebypermit formation of the particles 112. The maximum average streamtemperature, or reaction temperature, refers to the maximum averagetemperature that an aerosol stream attains while flowing through thefurnace. This is typically determined by a temperature probe insertedinto the furnace. Preferred reaction temperatures according to thepresent invention are discussed more fully below.

Although longer residence times are possible, for many applications,residence time in the heating zone of the furnace 110 of shorter thanabout 4 seconds is typical, with shorter than about 2 seconds beingpreferred, shorter than about 1 second being more preferred, shorterthan about 0.5 second being even more preferred, and shorter than about0.2 second being most preferred. The residence time should be longenough, however, to assure that the particles 112 attain the desiredmaximum stream temperature for a given heat transfer rate. In thatregard, with extremely short residence times, higher furnacetemperatures could be used to increase the rate of heat transfer so longas the particles 112 attain a maximum temperature within the desiredstream temperature range. That mode of operation, however, is notpreferred. Also, it is preferred that, in most cases, the maximum streamtemperature not be attained in the furnace 110 until substantially atthe end of the heating zone in the furnace 110. For example, the heatingzone will often include a plurality of heating sections that are eachindependently controllable. The maximum stream temperature shouldtypically not be attained until the final heating section, and morepreferably until substantially at the end of the last heating section.This is important to reduce the potential for thermophoretic losses ofmaterial. Also, it is noted that as used herein, residence time refersto the actual time for a material to pass through the relevant processequipment. In the case of the furnace, this includes the effect ofincreasing velocity with gas expansion due to heating.

Typically, the furnace 110 will be a tube-shaped furnace, so that theaerosol 108 moving into and through the furnace does not encounter sharpedges on which droplets could collect. Loss of droplets to collection atsharp surfaces results in a lower yield of particles 112. Moreimportant, however, the accumulation of liquid at sharp edges can resultin re-release of undesirably large droplets back into the aerosol 108,which can cause contamination of the particulate product 116 withundesirably large particles. Also, over time, such liquid collection atsharp surfaces can cause fouling of process equipment, impairing processperformance.

The furnace 110 may include a heating tube made of any suitablematerial. The tube material may be a ceramic material, for example,mullite, silica or alumina. Alternatively, the tube may be metallic.Advantages of using a metallic tube are low cost, ability to withstandsteep temperature gradients and large thermal shocks, machinability andweldability, and ease of providing a seal between the tube and otherprocess equipment. Disadvantages of using a metallic tube includelimited operating temperature and increased reactivity in some reactionsystems.

When a metallic tube is used in the furnace 110, it is preferably a highnickel content stainless steel alloy, such as a 330 stainless steel, ora nickel-based super alloy. As noted, one of the major advantages ofusing a metallic tube is that the tube is relatively easy to seal withother process equipment. In that regard, flange fittings may be weldeddirectly to the tube for connecting with other process equipment.Metallic tubes are generally preferred for making particles that do notrequire a maximum tube wall temperature of higher than about 1100° C.during particle manufacture.

When higher temperatures are required, ceramic tubes are typically used.One major problem with ceramic tubes, however, is that the tubes can bedifficult to seal with other process equipment, especially when the endsof the tubes are maintained at relatively high temperatures, as is oftenthe case with the present invention.

One configuration for sealing a ceramic tube is shown in FIGS. 2, 3 and4. The furnace 110 includes a ceramic tube 374 having an end cap 376fitted to each end of the tube 374, with a gasket 378 disposed betweencorresponding ends of the ceramic tube 374 and the end caps 376. Thegasket may be of any suitable material for sealing at the temperatureencountered at the ends of the tubes 374. Examples of gasket materialsfor sealing at temperatures below about 250° C. include silicone,TEFLON™ and VITON™. Examples of gasket materials for higher temperaturesinclude graphite, ceramic paper, thin sheet metal, and combinationsthereof.

Tension rods 380 extend over the length of the furnace 110 and throughrod holes 382 through the end caps 376. The tension rods 380 are held intension by the force of springs 384 bearing against bearing plates 386and the end caps 376. The tube 374 is, therefore, in compression due tothe force of the springs 384. The springs 384 may be compressed anydesired amount to form a seal between the end caps 376 and the ceramictube 374 through the gasket 378. As will be appreciated, by using thesprings 384, the tube 374 is free to move to some degree as it expandsupon heating and contracts upon cooling. To form the seal between theend caps 376 and the ceramic tube 374, one of the gaskets 378 is seatedin a gasket seat 388 on the side of each end cap 376 facing the tube374. A mating face 390 on the side of each of the end caps 376 facesaway from the tube 374, for mating with a flange surface for connectionwith an adjacent piece of equipment.

Also, although the present invention is described with primary referenceto a furnace reactor, which is preferred, it should be recognized that,except as noted, any other thermal reactor, including a flame reactor ora plasma reactor, could be used instead. A furnace reactor is, however,preferred, because of the generally even heating characteristic of afurnace for attaining a uniform stream temperature.

The particle collector 114, may be any suitable apparatus for collectingparticles 112 to produce the particulate product 116. One preferredembodiment of the particle collector 114 uses one or more filter toseparate the particles 112 from gas. Such a filter may be of any type,including a bag filter. Another preferred embodiment of the particlecollector uses one or more cyclone to separate the particles 112. Otherapparatus that may be used in the particle collector 114 includes anelectrostatic precipitator. Also, collection should normally occur at atemperature above the condensation temperature of the gas stream inwhich the particles 112 are suspended. Also, collection should normallybe at a temperature that is low enough to prevent significantagglomeration of the particles 112.

Of significant importance to the operation of the process of the presentinvention is the aerosol generator 106, which must be capable ofproducing a high quality aerosol with high droplet loading, aspreviously noted. With reference to FIG. 5, one embodiment of an aerosolgenerator 106 of the present invention is described. The aerosolgenerator 106 includes a plurality of ultrasonic transducer discs 120that are each mounted in a transducer housing 122. The transducerhousings 122 are mounted to a transducer mounting plate 124, creating anarray of the ultrasonic transducer discs 120. Any convenient spacing maybe used for the ultrasonic transducer discs 120. Center-to-centerspacing of the ultrasonic transducer discs 120 of about 4 centimeters isoften adequate. The aerosol generator 106, as shown in FIG. 5, includesforty-nine transducers in a 7×7 array. The array configuration is asshown in FIG. 6, which depicts the locations of the transducer housings122 mounted to the transducer mounting plate 124.

With continued reference to FIG. 5, a separator 126, in spaced relationto the transducer discs 120, is retained between a bottom retainingplate 128 and a top retaining plate 130. Gas delivery tubes 132 areconnected to gas distribution manifolds 134, which have gas deliveryports 136. The gas distribution manifolds 134 are housed within agenerator body 138 that is covered by generator lid 140. A transducerdriver 144, having circuitry for driving the transducer discs 120, iselectronically connected with the transducer discs 120 via electricalcables 146.

During operation of the aerosol generator 106, as shown in FIG. 5, thetransducer discs 120 are activated by the transducer driver 144 via theelectrical cables 146. The transducers preferably vibrate at a frequencyof from about 1 MHz to about 5 MHz, more preferably from about 1.5 MHzto about 3 MHz. Frequently used frequencies are at about 1.6 MHz andabout 2.4 MHz. Furthermore, all of the transducer discs 110 should beoperating at substantially the same frequency when an aerosol with anarrow droplet size distribution is desired. This is important becausecommercially available transducers can vary significantly in thickness,sometimes by as much as 10%. It is preferred, however, that thetransducer discs 120 operate at frequencies within a range of 5% aboveand below the median transducer frequency, more preferably within arange of 2.5%, and most preferably within a range of 1%. This can beaccomplished by careful selection of the transducer discs 120 so thatthey all preferably have thicknesses within 5% of the median transducerthickness, more preferably within 2.5%, and most preferably within 1%.

Liquid feed 102 enters through a feed inlet 148 and flows through flowchannels 150 to exit through feed outlet 152. An ultrasonicallytransmissive fluid, typically water, enters through a water inlet 154 tofill a water bath volume 156 and flow through flow channels 158 to exitthrough a water outlet 160. A proper flow rate of the ultrasonicallytransmissive fluid is necessary to cool the transducer discs 120 and toprevent overheating of the ultrasonically transmissive fluid. Ultrasonicsignals from the transducer discs 120 are transmitted, via theultrasonically transmissive fluid, across the water bath volume 156, andultimately across the separator 126, to the liquid feed 102 in flowchannels 150.

The ultrasonic signals from the ultrasonic transducer discs 120 causeatomization cones 162 to develop in the liquid feed 102 at locationscorresponding with the transducer discs 120. Carrier gas 104 isintroduced into the gas delivery tubes 132 and delivered to the vicinityof the atomization cones 162 via gas delivery ports 136. Jets of carriergas exit the gas delivery ports 136 in a direction so as to impinge onthe atomization cones 162, thereby sweeping away atomized droplets ofthe liquid feed 102 that are being generated from the atomization cones162 and creating the aerosol 108, which exits the aerosol generator 106through an aerosol exit opening 164.

Efficient use of the carrier gas 104 is an important aspect of theaerosol generator 106. The embodiment of the aerosol generator 106 shownin FIG. 5 includes two gas exit ports per atomization cone 162, with thegas ports being positioned above the liquid medium 102 over troughs thatdevelop between the atomization cones 162, such that the exiting carriergas 104 is horizontally directed at the surface of the atomization cones162, thereby efficiently distributing the carrier gas 104 to criticalportions of the liquid feed 102 for effective and efficient sweepingaway of droplets as they form about the ultrasonically energizedatomization cones 162. Furthermore, it is preferred that at least aportion of the opening of each of the gas delivery ports 136, throughwhich the carrier gas exits the gas delivery tubes, should be locatedbelow the top of the atomization cones 162 at which the carrier gas 104is directed. This relative placement of the gas delivery ports 136 isvery important to efficient use of carrier gas 104. Orientation of thegas delivery ports 136 is also important. Preferably, the gas deliveryports 136 are positioned to horizontally direct jets of the carrier gas104 at the atomization cones 162. The aerosol generator 106 permitsgeneration of the aerosol 108 with heavy loading with droplets of thecarrier liquid 102, unlike aerosol generator designs that do notefficiently focus gas delivery to the locations of droplet formation.

Another important feature of the aerosol generator 106, as shown in FIG.5, is the use of the separator 126, which protects the transducer discs120 from direct contact with the liquid feed 102, which is often highlycorrosive. The height of the separator 126 above the top of thetransducer discs 120 should normally be kept as small as possible, andis often in the range of from about 1 centimeter to about 2 centimeters.The top of the liquid feed 102 in the flow channels above the tops ofthe ultrasonic transducer discs 120 is typically in a range of fromabout 2 centimeters to about 5 centimeters, whether or not the aerosolgenerator includes the separator 126, with a distance of about 3 to 4centimeters being preferred. Although the aerosol generator 106 could bemade without the separator 126, in which case the liquid feed 102 wouldbe in direct contact with the transducer discs 120, the highly corrosivenature of the liquid feed 102 can often cause premature failure of thetransducer discs 120. The use of the separator 126, in combination withuse of the ultrasonically transmissive fluid in the water bath volume156 to provide ultrasonic coupling, significantly extending the life ofthe ultrasonic transducers 120. One disadvantage of using the separator126, however, is that the rate of droplet production from theatomization cones 162 is reduced, often by a factor of two or more,relative to designs in which the liquid feed 102 is in direct contactwith the ultrasonic transducer discs 102. Even with the separator 126,however, the aerosol generator 106 used with the present invention iscapable of producing a high quality aerosol with heavy droplet loading,as previously discussed. Suitable materials for the separator 126include, for example, polyamides (such as Kapton™ membranes from DuPont)and other polymer materials, glass, and plexiglass. The mainrequirements for the separator 126 are that it be ultrasonicallytransmissive, corrosion resistant and impermeable.

One alternative to using the separator 126 is to bind acorrosion-resistant protective coating onto the surface of theultrasonic transducer discs 120, thereby preventing the liquid feed 102from contacting the surface of the ultrasonic transducer discs 120. Whenthe ultrasonic transducer discs 120 have a protective coating, theaerosol generator 106 will typically be constructed without the waterbath volume 156 and the liquid feed 102 will flow directly over theultrasonic transducer discs 120. Examples of such protective coatingmaterials include platinum, gold, TEFLON™, epoxies and various plastics.Such coating typically significantly extends transducer life. Also, whenoperating without the separator 126, the aerosol generator 106 willtypically produce the aerosol 108 with a much higher droplet loadingthan when the separator 126 is used.

One surprising finding with operation of the aerosol generator 106 ofthe present invention is that the droplet loading in the aerosol may beaffected by the temperature of the liquid feed 102. It has been foundthat when the liquid feed 102 includes an aqueous liquid at an elevatedtemperature, the droplet loading increases significantly. Thetemperature of the liquid feed 102 is preferably higher than about 30°C., more preferably higher than about 35° C. and most preferably higherthan about 40° C. If the temperature becomes too high, however, it canhave a detrimental effect on droplet loading in the aerosol 108.Therefore, the temperature of the liquid feed 102 from which the aerosol108 is made should generally be lower than about 50° C., and preferablylower than about 45° C. The liquid feed 102 may be maintained at thedesired temperature in any suitable fashion. For example, the portion ofthe aerosol generator 106 where the liquid feed 102 is converted to theaerosol 108 could be maintained at a constant elevated temperature.Alternatively, the liquid feed 102 could be delivered to the aerosolgenerator 106 from a constant temperature bath maintained separate fromthe aerosol generator 106. When the ultrasonic generator 106 includesthe separator 126, the ultrasonically transmissive fluid adjacent theultrasonic transducer disks 120 are preferably also at an elevatedtemperature in the ranges just discussed for the liquid feed 102.

The design for the aerosol generator 106 based on an array of ultrasonictransducers is versatile and is easily modified to accommodate differentgenerator sizes for different specialty applications. The aerosolgenerator 106 may be designed to include a plurality of ultrasonictransducers in any convenient number. Even for smaller scale production,however, the aerosol generator 106 preferably has at least nineultrasonic transducers, more preferably at least 16 ultrasonictransducers, and even more preferably at least 25 ultrasonictransducers. For larger scale production, however, the aerosol generator106 includes at least 40 ultrasonic transducers, more preferably atleast 100 ultrasonic transducers, and even more preferably at least 400ultrasonic transducers. In some large volume applications, the aerosolgenerator may have at least 1000 ultrasonic transducers.

FIGS. 7–24 show component designs for an aerosol generator 106 includingan array of 400 ultrasonic transducers. Referring first to FIGS. 7 and8, the transducer mounting plate 124 is shown with a design toaccommodate an array of 400 ultrasonic transducers, arranged in foursubarrays of 100 ultrasonic transducers each. The transducer mountingplate 124 has integral vertical walls 172 for containing theultrasonically transmissive fluid, typically water, in a water bathsimilar to the water bath volume 156 described previously with referenceto FIG. 5.

As shown in FIGS. 7 and 8, four hundred transducer mounting receptacles174 are provided in the transducer mounting plate 124 for mountingultrasonic transducers for the desired array. With reference to FIG. 9,the profile of an individual transducer mounting receptacle 174 isshown. A mounting seat 176 accepts an ultrasonic transducer formounting, with a mounted ultrasonic transducer being held in place viascrew holes 178. Opposite the mounting receptacle 176 is a flaredopening 180 through which an ultrasonic signal may be transmitted forthe purpose of generating the aerosol 108, as previously described withreference to FIG. 5.

A preferred transducer mounting configuration, however, is shown in FIG.10 for another configuration for the transducer mounting plate 124. Asseen in FIG. 10, an ultrasonic transducer disc 120 is mounted to thetransducer mounting plate 124 by use of a compression screw 177 threadedinto a threaded receptacle 179. The compression screw 177 bears againstthe ultrasonic transducer disc 120, causing an o-ring 181, situated inan o-ring seat 182 on the transducer mounting plate, to be compressed toform a seal between the transducer mounting plate 124 and the ultrasonictransducer disc 120. This type of transducer mounting is particularlypreferred when the ultrasonic transducer disc 120 includes a protectivesurface coating, as discussed previously, because the seal of the o-ringto the ultrasonic transducer disc 120 will be inside of the outer edgeof the protective seal, thereby preventing liquid from penetrating underthe protective surface coating from the edges of the ultrasonictransducer disc 120.

Referring now to FIG. 11, the bottom retaining plate 128 for a 400transducer array is shown having a design for mating with the transducermounting plate 124 (shown in FIGS. 7–8). The bottom retaining plate 128has eighty openings 184, arranged in four subgroups 186 of twentyopenings 184 each. Each of the openings 184 corresponds with five of thetransducer mounting receptacles 174 (shown in FIGS. 7 and 8) when thebottom retaining plate 128 is mated with the transducer mounting plate124 to create a volume for a water bath between the transducer mountingplate 124 and the bottom retaining plate 128. The openings 184,therefore, provide a pathway for ultrasonic signals generated byultrasonic transducers to be transmitted through the bottom retainingplate.

Referring now to FIGS. 12 and 13, a liquid feed box 190 for a 400transducer array is shown having the top retaining plate 130 designed tofit over the bottom retaining plate 128 (shown in FIG. 11), with aseparator 126 (not shown) being retained between the bottom retainingplate 128 and the top retaining plate 130 when the aerosol generator 106is assembled. The liquid feed box 190 also includes vertically extendingwalls 192 for containing the liquid feed 102 when the aerosol generatoris in operation. Also shown in FIGS. 12 and 13 is the feed inlet 148 andthe feed outlet 152. An adjustable weir 198 determines the level ofliquid feed 102 in the liquid feed box 190 during operation of theaerosol generator 106.

The top retaining plate 130 of the liquid feed box 190 has eightyopenings 194 therethrough, which are arranged in four subgroups 196 oftwenty openings 194 each. The openings 194 of the top retaining plate130 correspond in size with the openings 184 of the bottom retainingplate 128 (shown in FIG. 11). When the aerosol generator 106 isassembled, the openings 194 through the top retaining plate 130 and theopenings 184 through the bottom retaining plate 128 are aligned, withthe separator 126 positioned therebetween, to permit transmission ofultrasonic signals when the aerosol generator 106 is in operation.

Referring now to FIGS. 12–14, a plurality of gas tube feed-through holes202 extend through the vertically extending walls 192 to either side ofthe assembly including the feed inlet 148 and feed outlet 152 of theliquid feed box 190. The gas tube feed-through holes 202 are designed topermit insertion therethrough of gas tubes 208 of a design as shown inFIG. 14. When the aerosol generator 106 is assembled, a gas tube 208 isinserted through each of the gas tube feed-through holes 202 so that gasdelivery ports 136 in the gas tube 208 will be properly positioned andaligned adjacent the openings 194 in the top retaining plate 130 fordelivery of gas to atomization cones that develop in the liquid feed box190 during operation of the aerosol generator 106. The gas deliveryports 136 are typically holes having a diameter of from about 1.5millimeters to about 3.5 millimeters.

Referring now to FIG. 15, a partial view of the liquid feed box 190 isshown with gas tubes 208A, 208B and 208C positioned adjacent to theopenings 194 through the top retaining plate 130. Also shown in FIG. 15are the relative locations that ultrasonic transducer discs 120 wouldoccupy when the aerosol generator 106 is assembled. As seen in FIG. 15,the gas tube 208A, which is at the edge of the array, has five gasdelivery ports 136. Each of the gas delivery ports 136 is positioned todivert carrier gas 104 to a different one of atomization cones thatdevelop over the array of ultrasonic transducer discs 120 when theaerosol generator 106 is operating. The gas tube 208B, which is one rowin from the edge of the array, is a shorter tube that has ten gasdelivery ports 136, five each on opposing sides of the gas tube 208B.The gas tube 208B, therefore, has gas delivery ports 136 for deliveringgas to atomization cones corresponding with each of ten ultrasonictransducer discs 120. The third gas tube, 208C, is a longer tube thatalso has ten gas delivery ports 136 for delivering gas to atomizationcones corresponding with ten ultrasonic transducer discs 120. The designshown in FIG. 15, therefore, includes one gas delivery port perultrasonic transducer disc 120. Although this is a lower density of gasdelivery ports 136 than for the embodiment of the aerosol generator 106shown in FIG. 5, which includes two gas delivery ports per ultrasonictransducer disc 120, the design shown in FIG. 15 is, nevertheless,capable of producing a dense, high-quality aerosol without unnecessarywaste of gas.

Referring now to FIG. 16, the flow of carrier gas 104 relative toatomization cones 162 during operation of the aerosol generator 106having a gas distribution configuration to deliver carrier gas 104 fromgas delivery ports on both sides of the gas tubes 208, as was shown forthe gas tubes 208A, 208B and 208C in the gas distribution configurationshown in FIG. 14. The carrier gas 104 sweeps both directions from eachof the gas tubes 208.

An alternative, and preferred, flow for carrier gas 104 is shown in FIG.17. As shown in FIG. 17, carrier gas 104 is delivered from only one sideof each of the gas tubes 208. This results in a sweep of carrier gasfrom all of the gas tubes 208 toward a central area 212. This results ina more uniform flow pattern for aerosol generation that maysignificantly enhance the efficiency with which the carrier gas 104 isused to produce an aerosol. The aerosol that is generated, therefore,tends to be more heavily loaded with liquid droplets.

Another configuration for distributing carrier gas in the aerosolgenerator 106 is showing in FIGS. 18 and 19. In this configuration, thegas tubes 208 are hung from a gas distribution plate 216 adjacent gasflow holes 218 through the gas distribution plate 216. In the aerosolgenerator 106, the gas distribution plate 216 would be mounted above theliquid feed, with the gas flow holes positioned to each correspond withan underlying ultrasonic transducer. Referring specifically to FIG. 19,when the ultrasonic generator 106 is in operation, atomization cones 162develop through the gas flow holes 218, and the gas tubes 208 arelocated such that carrier gas 104 exiting from ports in the gas tubes208 impinge on the atomization cones and flow upward through the gasflow holes. The gas flow holes 218, therefore, act to assist inefficiently distributing the carrier gas 104 about the atomization cones162 for aerosol formation. It should be appreciated that the gasdistribution plates 218 can be made to accommodate any number of the gastubes 208 and gas flow holes 218. For convenience of illustration, theembodiment shown in FIGS. 18 and 19 shows a design having only two ofthe gas tubes 208 and only 16 of the gas flow holes 218. Also, it shouldbe appreciated that the gas distribution plate 216 could be used alone,without the gas tubes 208. In that case, a slight positive pressure ofcarrier gas 104 would be maintained under the gas distribution plate 216and the gas flow holes 218 would be sized to maintain the propervelocity of carrier gas 104 through the gas flow holes 218 for efficientaerosol generation. Because of the relative complexity of operating inthat mode, however, it is not preferred.

Aerosol generation may also be enhanced through mounting of ultrasonictransducers at a slight angle and directing the carrier gas at resultingatomization cones such that the atomization cones are tilting in thesame direction as the direction of flow of carrier gas. Referring toFIG. 20, an ultrasonic transducer disc 120 is shown. The ultrasonictransducer disc 120 is tilted at a tilt angle 114 (typically less than10 degrees), so that the atomization cone 162 will also have a tilt. Itis preferred that the direction of flow of the carrier gas 104 directedat the atomization cone 162 is in the same direction as the tilt of theatomization cone 162.

Referring now to FIGS. 21 and 22, a gas manifold 220 is shown fordistributing gas to the gas tubes 208 in a 400 transducer array design.The gas manifold 220 includes a gas distribution box 222 and pipingstubs 224 for connection with gas tubes 208 (shown in FIG. 14). Insidethe gas distribution box 222 are two gas distribution plates 226 thatform a flow path to assist in distributing the gas equally throughoutthe gas distribution box 222, to promote substantially equal delivery ofgas through the piping stubs 224. The gas manifold 220, as shown inFIGS. 21 and 22, is designed to feed eleven gas tubes 208. For the 400transducer design, a total of four gas manifolds 220 are required.

Referring now to FIGS. 23 and 24, the generator lid 140 is shown for a400 transducer array design. The generator lid 140 mates with and coversthe liquid feed box 190 (shown in FIGS. 12 and 13). The generator lid140, as shown in FIGS. 23 and 24, has a hood design to permit easycollection of the aerosol 108 without subjecting droplets in the aerosol108 to sharp edges on which droplets may coalesce and be lost, andpossibly interfere with the proper operation of the aerosol generator106. When the aerosol generator 106 is in operation, the aerosol 108would be withdrawn via the aerosol exit opening 164 through thegenerator cover 140.

Although the aerosol generator 106 produces a high quality aerosol 108having a high droplet loading, it is often desirable to furtherconcentrate the aerosol 108 prior to introduction into the furnace 110.Referring now to FIG. 25, a process flow diagram is shown for oneembodiment of the present invention involving such concentration of theaerosol 108. As shown in FIG. 25, the aerosol 108 from the aerosolgenerator 106 is sent to an aerosol concentrator 236 where excesscarrier gas 238 is withdrawn from the aerosol 108 to produce aconcentrated aerosol 240, which is then fed to the furnace 110.

The aerosol concentrator 236 typically includes one or more virtualimpactors capable of concentrating droplets in the aerosol 108 by afactor of greater than about 2, preferably by a factor of greater thanabout 5, and more preferably by a factor of greater than about 10, toproduce the concentrated aerosol 240. According to the presentinvention, the concentrated aerosol 240 should typically contain greaterthan about 1×10⁷ droplets per cubic centimeter, and more preferably fromabout 5×10⁷ to about 5×10⁸ droplets per cubic centimeter. Aconcentration of about 1×10⁸ droplets per cubic centimeter of theconcentrated aerosol is particularly preferred, because when theconcentrated aerosol 240 is loaded more heavily than that, then thefrequency of collisions between droplets becomes large enough to impairthe properties of the concentrated aerosol 240, resulting in potentialcontamination of the particulate product 116 with an undesirably largequantity of over-sized particles. For example, if the aerosol 108 has aconcentration of about 1×10⁷ droplets per cubic centimeter, and theaerosol concentrator 236 concentrates droplets by a factor of 10, thenthe concentrated aerosol 240 will have a concentration of about 1×10⁸droplets per cubic centimeter. Stated another way, for example, when theaerosol generator generates the aerosol 108 with a droplet loading ofabout 0.167 milliliters liquid feed 102 per liter of carrier gas 104,the concentrated aerosol 240 would be loaded with about 1.67 millilitersof liquid feed 102 per liter of carrier gas 104, assuming the aerosol108 is concentrated by a factor of 10.

Having a high droplet loading in aerosol feed to the furnace providesthe important advantage of reducing the heating demand on the furnace110 and the size of flow conduits required through the furnace. Also,other advantages of having a dense aerosol include a reduction in thedemands on cooling and particle collection components, permittingsignificant equipment and operational savings. Furthermore, as systemcomponents are reduced in size, powder holdup within the system isreduced, which is also desirable. Concentration of the aerosol streamprior to entry into the furnace 110, therefore, provides a substantialadvantage relative to processes that utilize less concentrated aerosolstreams.

The excess carrier gas 238 that is removed in the aerosol concentrator236 typically includes extremely small droplets that are also removedfrom the aerosol 108. Preferably, the droplets removed with the excesscarrier gas 238 have a weight average size of smaller than about 1.5 μm,and more preferably smaller than about 1 μm and the droplets retained inthe concentrated aerosol 240 have an average droplet size of larger thanabout 2 μm. For example, a virtual impactor sized to treat an aerosolstream having a weight average droplet size of about three μm might bedesigned to remove with the excess carrier gas 238 most droplets smallerthan about 1.5 μm in size. Other designs are also possible. When usingthe aerosol generator 106 with the present invention, however, the lossof these very small droplets in the aerosol concentrator 236 willtypically constitute no more than about 10 percent by weight, and morepreferably no more than about 5 percent by weight, of the dropletsoriginally in the aerosol stream that is fed to the concentrator 236.Although the aerosol concentrator 236 is useful in some situations, itis normally not required with the process of the present invention,because the aerosol generator 106 is capable, in most circumstances, ofgenerating an aerosol stream that is sufficiently dense. So long as theaerosol stream coming out of the aerosol generator 102 is sufficientlydense, it is preferred that the aerosol concentrator not be used. It isa significant advantage of the present invention that the aerosolgenerator 106 normally generates such a dense aerosol stream that theaerosol concentrator 236 is not needed. Therefore, the complexity ofoperation of the aerosol concentrator 236 and accompanying liquid lossesmay typically be avoided.

It is important that the aerosol stream (whether it has beenconcentrated or not) that is fed to the furnace 110 have a high dropletflow rate and high droplet loading as would be required for mostindustrial applications. With the present invention, the aerosol streamfed to the furnace preferably includes a droplet flow of greater thanabout 0.5 liters per hour, more preferably greater than about 2 litersper hour, still more preferably greater than about 5 liters per hour,even more preferably greater than about 10 liters per hour, particularlygreater than about 50 liters per hour and most preferably greater thanabout 100 liters per hour; and with the droplet loading being typicallygreater than about 0.04 milliliters of droplets per liter of carriergas, preferably greater than about 0.083 milliliters of droplets perliter of carrier gas 104, more preferably greater than about 0.167milliliters of droplets per liter of carrier gas 104, still morepreferably greater than about 0.25 milliliters of droplets per liter ofcarrier gas 104, particularly greater than about 0.33 milliliters ofdroplets per liter of carrier gas 104 and most preferably greater thanabout 0.83 milliliters of droplets per liter of carrier gas 104.

One embodiment of a virtual impactor that could be used as the aerosolconcentrator 236 will now be described with reference to FIGS. 26–32. Avirtual impactor 246 includes an upstream plate assembly 248 (detailsshown in FIGS. 27–29) and a downstream plate assembly 250 (details shownin FIGS. 25–32), with a concentrating chamber 262 located between theupstream plate assembly 248 and the downstream plate assembly 250.

Through the upstream plate assembly 248 are a plurality of verticallyextending inlet slits 254. The downstream plate assembly 250 includes aplurality of vertically extending exit slits 256 that are in alignmentwith the inlet slits 254. The exit slits 256 are, however, slightlywider than the inlet slits 254. The downstream plate assembly 250 alsoincludes flow channels 258 that extend substantially across the width ofthe entire downstream plate assembly 250, with each flow channel 258being adjacent to an excess gas withdrawal port 260.

During operation, the aerosol 108 passes through the inlet slits 254 andinto the concentrating chamber 262. Excess carrier gas 238 is withdrawnfrom the concentrating chamber 262 via the excess gas withdrawal ports260. The withdrawn excess carrier gas 238 then exits via a gas duct port264. That portion of the aerosol 108 that is not withdrawn through theexcess gas withdrawal ports 260 passes through the exit slits 256 andthe flow channels 258 to form the concentrated aerosol 240. Thosedroplets passing across the concentrating chamber 262 and through theexit slits 256 are those droplets of a large enough size to havesufficient momentum to resist being withdrawn with the excess carriergas 238.

As seen best in FIGS. 27–32, the inlet slits 254 of the upstream plateassembly 248 include inlet nozzle extension portions 266 that extendoutward from the plate surface 268 of the upstream plate assembly 248.The exit slits 256 of the downstream plate assembly 250 include exitnozzle extension portions 270 extending outward from a plate surface 272of the downstream plate assembly 250. These nozzle extension portions266 and 270 are important for operation of the virtual impactor 246,because having these nozzle extension portions 266 and 270 permits avery close spacing to be attained between the inlet slits 254 and theexit slits 256 across the concentrating chamber 262, while alsoproviding a relatively large space in the concentrating chamber 262 tofacilitate efficient removal of the excess carrier gas 238.

Also as best seen in FIGS. 27–32, the inlet slits 254 have widths thatflare outward toward the side of the upstream plate assembly 248 that isfirst encountered by the aerosol 108 during operation. This flaredconfiguration reduces the sharpness of surfaces encountered by theaerosol 108, reducing the loss of aerosol droplets and potentialinterference from liquid buildup that could occur if sharp surfaces werepresent. Likewise, the exit slits 256 have a width that flares outwardtowards the flow channels 258, thereby allowing the concentrated aerosol240 to expand into the flow channels 258 without encountering sharpedges that could cause problems.

As noted previously, both the inlet slits 254 of the upstream plateassembly 248 and the exit slits 256 of the downstream plate assembly 250are vertically extending. This configuration is advantageous forpermitting liquid that may collect around the inlet slits 254 and theexit slits 256 to drain away. The inlet slits 254 and the exit slits 256need not, however, have a perfectly vertical orientation. Rather, it isoften desirable to slant the slits backward (sloping upward and away inthe direction of flow) by about five to ten degrees relative tovertical, to enhance draining of liquid off of the upstream plateassembly 248 and the downstream plate assembly 250. This drainagefunction of the vertically extending configuration of the inlet slits254 and the outlet slits 256 also inhibits liquid build-up in thevicinity of the inlet slits 248 and the exit slits 250, which liquidbuild-up could result in the release of undesirably large droplets intothe concentrated aerosol 240.

As discussed previously, the aerosol generator 106 of the presentinvention produces a concentrated, high quality aerosol of micro-sizeddroplets having a relatively narrow size distribution. It has beenfound, however, that for many applications the process of the presentinvention is significantly enhanced by further classifying by size thedroplets in the aerosol 108 prior to introduction of the droplets intothe furnace 110. In this manner, the size and size distribution ofparticles in the particulate product 116 are further controlled.

Referring now to FIG. 33, a process flow diagram is shown for oneembodiment of the process of the present invention including suchdroplet classification. As shown in FIG. 33, the aerosol 108 from theaerosol generator 106 goes to a droplet classifier 280 where oversizeddroplets are removed from the aerosol 108 to prepare a classifiedaerosol 282. Liquid 284 from the oversized droplets that are beingremoved is drained from the droplet classifier 280. This drained liquid284 may advantageously be recycled for use in preparing additionalliquid feed 102.

Any suitable droplet classifier may be used for removing droplets abovea predetermined size. For example, a cyclone could be used to removeover-size droplets. A preferred droplet classifier for manyapplications, however, is an impactor. One embodiment of an impactor foruse with the present invention will now be described with reference toFIGS. 34–38.

As seen in FIG. 34, an impactor 288 has disposed in a flow conduit 286 aflow control plate 290 and an impactor plate assembly 292. The flowcontrol plate 290 is conveniently mounted on a mounting plate 294.

The flow control plate 290 is used to channel the flow of the aerosolstream toward the impactor plate assembly 292 in a manner withcontrolled flow characteristics that are desirable for proper impactionof oversize droplets on the impactor plate assembly 292 for removalthrough the drains 296 and 314. One embodiment of the flow control plate290 is shown in FIG. 35. The flow control plate 290 has an array ofcircular flow ports 296 for channeling flow of the aerosol 108 towardsthe impactor plate assembly 292 with the desired flow characteristics.

Details of the mounting plate 294 are shown in FIG. 36. The mountingplate 294 has a mounting flange 298 with a large diameter flow opening300 passing therethrough to permit access of the aerosol 108 to the flowports 296 of the flow control plate 290 (shown in FIG. 35).

Referring now to FIGS. 37 and 38, one embodiment of an impactor plateassembly 292 is shown. The impactor plate assembly 292 includes animpactor plate 302 and mounting brackets 304 and 306 used to mount theimpactor plate 302 inside of the flow conduit 286. The impactor plate302 and the flow channel plate 290 are designed so that droplets largerthan a predetermined size will have momentum that is too large for thoseparticles to change flow direction to navigate around the impactor plate302.

During operation of the impactor 288, the aerosol 108 from the aerosolgenerator 106 passes through the upstream flow control plate 290. Mostof the droplets in the aerosol navigate around the impactor plate 302and exit the impactor 288 through the downstream flow control plate 290in the classified aerosol 282. Droplets in the aerosol 108 that are toolarge to navigate around the impactor plate 302 will impact on theimpactor plate 302 and drain through the drain 296 to be collected withthe drained liquid 284 (as shown in FIG. 34).

The configuration of the impactor plate 302 shown in FIG. 33 representsonly one of many possible configurations for the impactor plate 302. Forexample, the impactor 288 could include an upstream flow control plate290 having vertically extending flow slits therethrough that are offsetfrom vertically extending flow slits through the impactor plate 302,such that droplets too large to navigate the change in flow due to theoffset of the flow slits between the flow control plate 290 and theimpactor plate 302 would impact on the impactor plate 302 to be drainedaway. Other designs are also possible.

In a preferred embodiment of the present invention, the dropletclassifier 280 is typically designed to remove droplets from the aerosol108 that are larger than about 15 μm in size, more preferably to removedroplets larger than about 10 μm in size, even more preferably to removedroplets of a size larger than about 8 μm in size and most preferably toremove droplets larger than about 5 μm in size. The dropletclassification size in the droplet classifier is preferably smaller thanabout 15 μm, more preferably smaller than about 10 μm, even morepreferably smaller than about 8 μm and most preferably smaller thanabout 5 μm. The classification size, also called the classification cutpoint, is that size at which half of the droplets of that size areremoved and half of the droplets of that size are retained. Dependingupon the specific application, however, the droplet classification sizemay be varied, such as by changing the spacing between the impactorplate 302 and the flow control plate 290 or increasing or decreasingaerosol velocity through the jets in the flow control plate 290. Becausethe aerosol generator 106 of the present invention initially produces ahigh quality aerosol 108, having a relatively narrow size distributionof droplets, typically less than about 30 weight percent of liquid feed102 in the aerosol 108 is removed as the drain liquid 284 in the dropletclassifier 288, with preferably less than about 25 weight percent beingremoved, even more preferably less than about 20 weight percent beingremoved and most preferably less than about 15 weight percent beingremoved. Minimizing the removal of liquid feed 102 from the aerosol 108is particularly important for commercial applications to increase theyield of high quality particulate product 116. It should be noted,however, that because of the superior performance of the aerosolgenerator 106, it is frequently not required to use an impactor or otherdroplet classifier to obtain a desired absence of oversize droplets tothe furnace. This is a major advantage, because the added complexity andliquid losses accompanying use of an impactor may often be avoided withthe process of the present invention.

Sometimes it is desirable to use both the aerosol concentrator 236 andthe droplet classifier 280 to produce an extremely high quality aerosolstream for introduction into the furnace for the production of particlesof highly controlled size and size distribution. Referring now to FIG.39, one embodiment of the present invention is shown incorporating boththe virtual impactor 246 and the impactor 288. Basic components of thevirtual impactor 246 and the impactor 288, as shown in FIG. 39, aresubstantially as previously described with reference to FIGS. 26–38. Asseen in FIG. 39, the aerosol 108 from the aerosol generator 106 is fedto the virtual impactor 246 where the aerosol stream is concentrated toproduce the concentrated aerosol 240. The concentrated aerosol 240 isthen fed to the impactor 288 to remove large droplets therefrom andproduce the classified aerosol 282, which may then be fed to the furnace110. Also, it should be noted that by using both a virtual impactor andan impactor, both undesirably large and undesirably small droplets areremoved, thereby producing a classified aerosol with a very narrowdroplet size distribution. Also, the order of the aerosol concentratorand the aerosol classifier could be reversed, so that the aerosolconcentrator 236 follows the aerosol classifier 280.

One important feature of the design shown in FIG. 39 is theincorporation of drains 310, 312, 314, 316 and 296 at strategiclocations. These drains are extremely important for industrial-scaleparticle production because buildup of liquid in the process equipmentcan significantly impair the quality of the particulate product 116 thatis produced. In that regard, drain 310 drains liquid away from the inletside of the first plate assembly 248 of the virtual impactor 246. Drain312 drains liquid away from the inside of the concentrating chamber 262in the virtual impactor 246 and drain 314 removes liquid that depositsout of the excess carrier gas 238. Drain 316 removes liquid from thevicinity of the inlet side of the flow control plate 290 of theimpactor, while the drain 296 removes liquid from the vicinity of theimpactor plate 302. Without these drains 310, 312, 314, 316 and 296, theperformance of the apparatus shown in FIG. 39 would be significantlyimpaired. All liquids drained in the drains 310, 312, 314, 316 and 296may advantageously be recycled for use to prepare the liquid feed 102.

With some applications of the process of the present invention, it maybe possible to collect the particles 112 directly from the output of thefurnace 110. More often, however, it will be desirable to cool theparticles 112 exiting the furnace 110 prior to collection of theparticles 112 in the particle collector 114. Referring now to FIG. 40,one embodiment of the process of the present invention is shown in whichthe particles 112 exiting the furnace 110 are sent to a particle cooler320 to produce a cooled particle stream 322, which is then feed to theparticle collector 114. Although the particle cooler 320 may be anycooling apparatus capable of cooling the particles 112 to the desiredtemperature for introduction into the particle collector 114,traditional heat exchanger designs are not preferred. This is because atraditional heat exchanger design ordinarily directly subjects theaerosol stream, in which the hot particles 112 are suspended, to coolsurfaces. In that situation, significant losses of the particles 112occur due to thermophoretic deposition of the hot particles 112 on thecool surfaces of the heat exchanger. According to the present invention,a gas quench apparatus is provided for use as the particle cooler 320that significantly reduces thermophoretic losses compared to atraditional heat exchanger.

Referring now to FIGS. 41–43, one embodiment of a gas quench cooler 330is shown. The gas quench cooler includes a perforated conduit 332 housedinside of a cooler housing 334 with an annular space 336 located betweenthe cooler housing 334 and the perforated conduit 332. In fluidcommunication with the annular space 336 is a quench gas inlet box 338,inside of which is disposed a portion of an aerosol outlet conduit 340.The perforated conduit 332 extends between the aerosol outlet conduit340 and an aerosol inlet conduit 342. Attached to an opening into thequench gas inlet box 338 are two quench gas feed tubes 344. Referringspecifically to FIG. 43, the perforated tube 332 is shown. Theperforated tube 332 has a plurality of openings 345. The openings 345,when the perforated conduit 332 is assembled into the gas quench cooler330, permit the flow of quench gas 346 from the annular space 336 intothe interior space 348 of the perforated conduit 332. Although theopenings 345 are shown as being round holes, any shape of opening couldbe used, such as slits. Also, the perforated conduit 332 could be aporous screen. Two heat radiation shields 347 prevent downstream radiantheating from the furnace. In most instances, however, it will not benecessary to include the heat radiation shields 347, because downstreamradiant heating from the furnace is normally not a significant problem.Use of the heat radiation shields 347 is not preferred due toparticulate losses that accompany their use.

With continued reference to FIGS. 41–43, operation of the gas quenchcooler 330 will now be described. During operation, the particles 112,carried by and dispersed in a gas stream, enter the gas quench cooler330 through the aerosol inlet conduit 342 and flow into the interiorspace 348 of perforated conduit 332. Quench gas 346 is introducedthrough the quench gas feed tubes 344 into the quench gas inlet box 338.Quench gas 346 entering the quench gas inlet box 338 encounters theouter surface of the aerosol outlet conduit 340, forcing the quench gas346 to flow, in a spiraling, swirling manner, into the annular space336, where the quench gas 346 flows through the openings 345 through thewalls of the perforated conduit 332. Preferably, the gas 346 retainssome swirling motion even after passing into the interior space 348. Inthis way, the particles 112 are quickly cooled with low losses ofparticles to the walls of the gas quench cooler 330. In this manner, thequench gas 346 enters in a radial direction into the interior space 348of the perforated conduit 332 around the entire periphery, orcircumference, of the perforated conduit 332 and over the entire lengthof the perforated conduit 332. The cool quench gas 346 mixes with andcools the hot particles 112, which then exit through the aerosol outletconduit 340 as the cooled particle stream 322. The cooled particlestream 322 can then be sent to the particle collector 114 for particlecollection. The temperature of the cooled particle stream 322 iscontrolled by introducing more or less quench gas. Also, as shown inFIG. 41, the quench gas 346 is fed into the quench cooler 330 in counterflow to flow of the particles. Alternatively, the quench cooler could bedesigned so that the quench gas 346 is fed into the quench cooler inconcurrent flow with the flow of the particles 112. The μmount of quenchgas 346 fed to the gas quench cooler 330 will depend upon the specificmaterial being made and the specific operating conditions. The quantityof quench gas 346 used, however, must be sufficient to reduce thetemperature of the aerosol steam including the particles 112 to thedesired temperature. Typically, the particles 112 are cooled to atemperature at least below about 200° C., and often lower. The onlylimitation on how much the particles 112 are cooled is that the cooledparticle stream 322 must be at a temperature that is above thecondensation temperature for water as another condensible vapor in thestream. The temperature of the cooled particle stream 322 is often at atemperature of from about 50° C. to about 120° C.

Because of the entry of quench gas 346 into the interior space 348 ofthe perforated conduit 322 in a radial direction about the entirecircumference and length of the perforated conduit 322, a buffer of thecool quench gas 346 is formed about the inner wall of the perforatedconduit 332, thereby significantly inhibiting the loss of hot particles112 due to thermophoretic deposition on the cool wall of the perforatedconduit 332. In operation, the quench gas 346 exiting the openings 345and entering into the interior space 348 should have a radial velocity(velocity inward toward the center of the circular cross-section of theperforated conduit 332) of larger than the thermophoretic velocity ofthe particles 112 inside the perforated conduit 332 in a directionradially outward toward the perforated wall of the perforated conduit332.

As seen in FIGS. 41–43, the gas quench cooler 330 includes a flow pathfor the particles 112 through the gas quench cooler of a substantiallyconstant cross-sectional shape and area. Preferably, the flow paththrough the gas quench cooler 330 will have the same cross-sectionalshape and area as the flow path through the furnace 110 and through theconduit delivering the aerosol 108 from the aerosol generator 106 to thefurnace 110. In one embodiment, however, it may be necessary to reducethe cross-sectional area available for flow prior to the particlecollector 114. This is the case, for example, when the particlecollector includes a cyclone for separating particles in the cooledparticle stream 322 from gas in the cooled particle stream 322. This isbecause of the high inlet velocity requirements into cyclone separators.

Referring now to FIG. 44, one embodiment of the gas quench cooler 330 isshown in combination with a cyclone separator 392. The perforatedconduit 332 has a continuously decreasing cross-sectional area for flowto increase the velocity of flow to the proper value for the feed tocyclone separator 392. Attached to the cyclone separator 392 is a bagfilter 394 for final clean-up of overflow from the cyclone separator392. Separated particles exit with underflow from the cyclone separator392 and may be collected in any convenient container. The use of cycloneseparation is particularly preferred for powder having a weight averagesize of larger than about 1 μm, although a series of cyclones maysometimes be needed to get the desired degree of separation. Cycloneseparation is particularly preferred for powders having a weight averagesize of larger than about 1.5 μm. Also, cyclone separation is bestsuited for high density materials. Preferably, when particles areseparated using a cyclone, the particles are of a composition withspecific gravity of greater than about 5.

In an additional embodiment, the process of the present invention canalso incorporate compositional modification of the particles 112 exitingthe furnace. Most commonly, the compositional modification will involveforming on the particles 112 a material phase that is different thanthat of the particles 112, such as by coating the particles 112 with acoating material. One embodiment of the process of the present inventionincorporating particle coating is shown in FIG. 45. As shown in FIG. 45,the particles 112 exiting from the furnace 110 go to a particle coater350 where a coating is placed over the outer surface of the particles112 to form coated particles 352, which are then sent to the particlecollector 114 for preparation of the particulate product 116. Coatingmethodologies employed in the particle coater 350 are discussed in moredetail below.

With continued reference primarily to FIG. 45, in a preferredembodiment, when the particles 112 are coated according to the processof the present invention, the particles 112 are also manufactured viathe aerosol process of the present invention, as previously described.The process of the present invention can, however, be used to coatparticles that have been premanufactured by a different process, such asby a liquid precipitation route. When coating particles that have beenpremanufactured by a different route, such as by liquid precipitation,it is preferred that the particles remain in a dispersed state from thetime of manufacture to the time that the particles are introduced inslurry form into the aerosol generator 106 for preparation of theaerosol 108 to form the dry particles 112 in the furnace 110, whichparticles 112 can then be coated in the particle coater 350. Maintainingparticles in a dispersed state from manufacture through coating avoidsproblems associated with agglomeration and redispersion of particles ifparticles must be redispersed in the liquid feed 102 for feed to theaerosol generator 106. For example, for particles originallyprecipitated from a liquid medium, the liquid medium containing thesuspended precipitated particles could be used to form the liquid feed102 to the aerosol generator 106. It should be noted that the particlecoater 350 could be an integral extension of the furnace 110 or could bea separate piece of equipment.

In a further embodiment of the present invention, following preparationof the particles 112 in the furnace 110, the particles 112 may then bestructurally modified to impart desired physical properties prior toparticle collection. Referring now to FIG. 46, one embodiment of theprocess of the present invention is shown including such structuralparticle modification. The particles 112 exiting the furnace 110 go to aparticle modifier 360 where the particles are structurally modified toform modified particles 362, which are then sent to the particlecollector 114 for preparation of the particulate product 116. Theparticle modifier 360 is typically a furnace, such as an annealingfurnace, which may be integral with the furnace 110 or may be a separateheating device. Regardless, it is important that the particle modifier360 have temperature control that is independent of the furnace 110, sothat the proper conditions for particle modification may be providedseparate from conditions required of the furnace 110 to prepare theparticles 112. The particle modifier 360, therefore, typically providesa temperature controlled environment and necessary residence time toeffect the desired structural modification of the particles 112.

The structural modification that occurs in the particle modifier 360 maybe any modification to the crystalline structure or morphology of theparticles 112. For example, the particles 112 may be annealed in theparticle modifier 360 to densify the particles 112 or to recrystallizethe particles 112 into a polycrystalline or single crystalline form.Also, especially in the case of composite particles 112, the particlesmay be annealed for a sufficient time to permit redistribution withinthe particles 112 of different material phases. Particularly preferredparameters for such processes are discussed in more detail below.

The initial morphology of composite particles made in the furnace 110,according to the present invention, could take a variety of forms,depending upon the specified materials involved and the specificprocessing conditions. Examples of some possible composite particlemorphologies, manufacturable according to the present invention areshown in FIG. 47. These morphologies could be of the particles asinitially produced in the furnace 110 or that result from structuralmodification in the particle modifier 360. Furthermore, the compositeparticles could include a mixture of the morphological attributes shownin FIG. 47.

Referring now to FIG. 48, an embodiment of the apparatus of the presentinvention is shown that includes the aerosol generator 106 (in the formof the 400 transducer array design), the aerosol concentrator 236 (inthe form of a virtual impactor), the droplet classifier 280 (in the formof an impactor), the furnace 110, the particle cooler 320 (in the formof a gas quench cooler) and the particle collector 114 (in the form of abag filter). All process equipment components are connected viaappropriate flow conduits that are substantially free of sharp edgesthat could detrimentally cause liquid accumulations in the apparatus.Also, it should be noted that flex connectors 370 are used upstream anddownstream of the aerosol concentrator 236 and the droplet classifier280. By using the flex connectors 370, it is possible to vary the angleof slant of vertically extending slits in the aerosol concentrator 236and/or the droplet classifier 280. In this way, a desired slant for thevertically extending slits may be set to optimize the drainingcharacteristics off the vertically extending slits.

Aerosol generation with the process of the present invention has thusfar been described with respect to the ultrasonic aerosol generator. Useof the ultrasonic generator is preferred for the process of the presentinvention because of the extremely high quality and dense aerosolgenerated. In some instances, however, the aerosol generation for theprocess of the present invention may have a different design dependingupon the specific application. For example, when larger particles aredesired, such as those having a weight average size of larger than about3 μm, a spray nozzle atomizer may be preferred. For smaller-particleapplications, however, and particularly for those applications toproduce particles smaller than about 3 μm, and preferably smaller thanabout 2 μm in size, as is generally desired with the particles of thepresent invention, the ultrasonic generator, as described herein, isparticularly preferred. In that regard, the ultrasonic generator of thepresent invention is particularly preferred for when making particleswith a weight average size of from about 0.2 μm to about 3 μm.

Although ultrasonic aerosol generators have been used for medicalapplications and home humidifiers, use of ultrasonic generators forspray pyrolysis particle manufacture has largely been confined tosmall-scale, experimental situations. The ultrasonic aerosol generatorof the present invention described with reference to FIGS. 5–24,however, is well suited for commercial production of high qualitypowders with a small average size and a narrow size distribution. Inthat regard, the aerosol generator produces a high quality aerosol, withheavy droplet loading and at a high rate of production. Such acombination of small droplet size, narrow size distribution, heavydroplet loading, and high production rate provide significant advantagesover existing aerosol generators that usually suffer from at least oneof inadequately narrow size distribution, undesirably low dropletloading, or unacceptably low production rate.

Through the careful and controlled design of the ultrasonic generator ofthe present invention, an aerosol may be produced typically havinggreater than about 70 weight percent (and preferably greater than about80 weight percent) of droplets in the size range of from about 1 μm toabout 10 μm, preferably in a size range of from about 1 μm to about 5 μmand more preferably from about 2 μm to about 4 μm. Also, the ultrasonicgenerator of the present invention is capable of delivering high outputrates of liquid feed in the aerosol. The rate of liquid feed, at thehigh liquid loadings previously described, is preferably greater thanabout 25 milliliters per hour per transducer, more preferably greaterthan about 37.5 milliliters per hour per transducer, even morepreferably greater than about 50 milliliters per hour per transducer andmost preferably greater than about 100 millimeters per hour pertransducer. This high level of performance is desirable for commercialoperations and is accomplished with the present invention with arelatively simple design including a single precursor bath over an arrayof ultrasonic transducers. The ultrasonic generator is made for highaerosol production rates at a high droplet loading, and with a narrowsize distribution of droplets. The generator preferably produces anaerosol at a rate of greater than about 0.5 liter per hour of droplets,more preferably greater than about 2 liters per hour of droplets, stillmore preferably greater than about 5 liters per hour of droplets, evenmore preferably greater than about 10 liters per hour of droplets andmost preferably greater than about 40 liters per hour of droplets. Forexample, when the aerosol generator has a 400 transducer design, asdescribed with reference to FIGS. 7–24, the aerosol generator is capableof producing a high quality aerosol having high droplet loading aspreviously described, at a total production rate of preferably greaterthan about 10 liters per hour of liquid feed, more preferably greaterthan about 15 liters per hour of liquid feed, even more preferablygreater than about 20 liters per hour of liquid feed and most preferablygreater than about 40 liters per hour of liquid feed.

Under most operating conditions, when using such an aerosol generator,total particulate product produced is preferably greater than about 0.5gram per hour per transducer, more preferably greater than about 0.75gram per hour per transducer, even more preferably greater than about1.0 gram per hour per transducer and most preferably greater than about2.0 grams per hour per transducer.

One significant aspect of the process of the present invention formanufacturing particulate materials is the unique flow characteristicsencountered in the furnace relative to laboratory scale systems. Themaximum Reynolds number attained for flow in the furnace 110 with thepresent invention is very high, typically in excess of 500, preferablyin excess of 1,000 and more preferably in excess of 2,000. In mostinstances, however, the maximum Reynolds number for flow in the furnacewill not exceed 10,000, and preferably will not exceed 5,000. This issignificantly different from lab-scale systems where the Reynolds numberfor flow in a reactor is typically lower than 50 and rarely exceeds 100.

The Reynolds number is a dimensionless quantity characterizing flow of afluid which, for flow through a circular cross sectional conduit isdefined as: ${Re} = \frac{\rho\; v\; d}{\mu}$where:

-   -   ρ=fluid density;    -   v=fluid mean velocity;    -   d=conduit inside diameter; and    -   μ=fluid viscosity.        It should be noted that the values for density, velocity and        viscosity will vary along the length of the furnace 110. The        maximum Reynolds number in the furnace 110 is typically attained        when the average stream temperature is at a maximum, because the        gas velocity is at a very high value due to gas expansion when        heated.

One problem with operating under flow conditions at a high Reynoldsnumber is that undesirable volatilization of components is much morelikely to occur than in systems having flow characteristics as found inlaboratory-scale systems. The volatilization problem occurs with thepresent invention, because the furnace is typically operated over asubstantial section of the heating zone in a constant wall heat fluxmode, due to limitations in heat transfer capability. This issignificantly different than operation of a furnace at a laboratoryscale, which typically involves operation of most of the heating zone ofthe furnace in a uniform wall temperature mode, because the heating loadis sufficiently small that the system is not heat transfer limited.

With the present invention, it is typically preferred to heat theaerosol stream in the heating zone of the furnace as quickly as possibleto the desired temperature range for particle manufacture. Because offlow characteristics in the-furnace and heat transfer limitations,during rapid heating of the aerosol the wall temperature of the furnacecan significantly exceed the maximum average target temperature for thestream. This is a problem because, even though the average streamtemperature may be within the range desired, the wall temperature maybecome so hot that components in the vicinity of the wall are subjectedto temperatures high enough to undesirably volatilize the components.This volatilization near the wall of the furnace can cause formation ofsignificant quantities of ultrafine particles that are outside of thesize range desired.

Therefore, with the present invention, it is preferred that when theflow characteristics in the furnace are such that the Reynolds numberthrough any part of the furnace exceeds 500, more preferably exceeds1,000, and most preferably exceeds 2,000, the maximum wall temperaturein the furnace should be kept at a temperature that is below thetemperature at which a desired component of the final particles wouldexert a vapor pressure not exceeding about 200 millitorr, morepreferably not exceeding about 100 millitorr, and most preferably notexceeding about 50 millitorr. Furthermore, the maximum wall temperaturein the furnace should also be kept below a temperature at which anintermediate component, from which a final component is to be at leastpartially derived, should also have a vapor pressure not exceeding themagnitudes noted for components of the final product.

In addition to maintaining the furnace wall temperature below a levelthat could create volatilization problems, it is also important thatthis not be accomplished at the expense of the desired average streamtemperature. The maximum average stream temperature must be maintainedat a high enough level so that the particles will have a desired highdensity. The maximum average stream temperature should, however,generally be a temperature at which a component in the final particles,or an intermediate component from which a component in the finalparticles is at least partially derived, would exert a vapor pressurenot exceeding about 100 millitorr, preferably not exceeding about 50millitorr, and most preferably not exceeding about 25 millitorr.

So long as the maximum wall temperature and the average streamtemperature are kept below the point at which detrimental volatilizationoccurs, it is generally desirable to heat the stream as fast as possibleand to remove resulting particles from the furnace immediately after themaximum stream temperature is reached in the furnace. With the presentinvention, the average residence time in the heating zone of the furnacemay typically be maintained at shorter than about 4 seconds, preferablyshorter than about 2 seconds, more preferably shorter than about 1second, still more preferably shorter than about 0.5 second, and mostpreferably shorter than about 0.2 second.

Another significant issue with respect to operating the process of thepresent invention, which includes high aerosol flow rates, is losswithin the system of materials intended for incorporation into the finalparticulate product. Material losses in the system can be quite high ifthe system is not properly operated. If system losses are too high, theprocess would not be practical for use in the manufacture of particulateproducts of many materials. This has typically not been a majorconsideration with laboratory-scale systems.

One significant potential for loss with the process of the presentinvention is thermophoretic losses that occur when a hot aerosol streamis in the presence of a cooler surface. In that regard, the use of thequench cooler, as previously described, with the process of the presentinvention provides an efficient way to cool the particles withoutunreasonably high thermophoretic losses. There is also, however,significant potential for losses occurring near the end of the furnaceand between the furnace and the cooling unit.

It has been found that thermophoretic losses in the back end of thefurnace can be significantly controlled if the heating zone of thefurnace is operated such that the maximum stream temperature is notattained until near the end of the heating zone in the furnace, and atleast not until the last third of the heating zone. When the heatingzone includes a plurality of heating sections, the maximum averagestream temperature should ordinarily not occur until at least the lastheating section. Furthermore, the heating zone should typically extendto as close to the exit of the furnace as possible. This is counter toconventional thought which is to typically maintain the exit portion ofthe furnace at a low temperature to avoid having to seal the furnaceoutlet at a high temperature. Such cooling of the exit portion of thefurnace, however, significantly promotes thermophoretic losses.Furthermore, the potential for operating problems that could result inthermophoretic losses at the back end of the furnace are reduced withthe very short residence times in the furnace for the present invention,as discussed previously.

Typically, it would be desirable to instantaneously cool the aerosolupon exiting the furnace. This is not possible. It is possible, however,to make the residence time between the furnace outlet and the coolingunit as short as possible. Furthermore, it is desirable to insulate theaerosol conduit occurring between the furnace exit and the cooling unitentrance. Even more preferred is to insulate that conduit and, even morepreferably, to also heat that conduit so that the wall temperature ofthat conduit is at least as high as the average stream temperature ofthe aerosol stream. Furthermore, it is desirable that the cooling unitoperate in a manner such that the aerosol is quickly cooled in a mannerto prevent thermophoretic losses during cooling. The quench cooler,described previously, is very effective for cooling with low losses.Furthermore, to keep the potential for thermophoretic losses very low,it is preferred that the residence time of the aerosol stream betweenattaining the maximum stream temperature in the furnace and a point atwhich the aerosol has been cooled to an average stream temperature belowabout 200° C. is shorter than about 2 seconds, more preferably shorterthan about 1 second, and even more preferably shorter than about 0.5second and most preferably shorter than about 0.1 second. In mostinstances, the maximum average stream temperature attained in thefurnace will be greater than about 800° C. Furthermore, the totalresidence time from the beginning of the heating zone in the furnace toa point at which the average stream temperature is at a temperaturebelow about 200° C. should typically be shorter than about 5 seconds,preferably shorter than about 3 seconds, more preferably shorter thanabout 2 seconds, and most preferably shorter than about 1 second.

Another part of the process with significant potential forthermophoretic losses is after particle cooling until the particles arefinally collected. Proper particle collection is very important toreducing losses within the system. The potential for thermophoreticlosses is significant following particle cooling because the aerosolstream is still at an elevated temperature to prevent detrimentalcondensation of water in the aerosol stream. Therefore, cooler surfacesof particle collection equipment can result in significantthermophoretic losses.

To reduce the potential for thermophoretic losses before the particlesare finally collected, it is important that the transition between thecooling unit and particle collection be as short as possible.Preferably, the output from the quench cooler is immediately sent to aparticle separator, such as a filter unit or a cyclone. In that regard,the total residence time of the aerosol between attaining the maximumaverage stream temperature in the furnace and the final collection ofthe particles is preferably shorter than about 2 seconds, morepreferably shorter than about 1 second, still more preferably shorterthan about 0.5 second and most preferably shorter than about 0.1 second.Furthermore, the residence time between the beginning of the heatingzone in the furnace and final collection of the particles is preferablyshorter than about 6 seconds, more preferably shorter than about 3seconds, even more preferably shorter than about 2 seconds, and mostpreferably shorter than about 1 second. Furthermore, the potential forthermophoretic losses may further be reduced by insulating the conduitsection between the cooling unit and the particle collector and, evenmore preferably, by also insulating around the filter, when a filter isused for particle collection. The potential for losses may be reducedeven further by heating of the conduit section between the cooling unitand the particle collection equipment, so that the internal equipmentsurfaces are at least slightly warmer than the aerosol stream averagestream temperature. Furthermore, when a filter is used for particlecollection, the filter could be heated. For example, insulation could bewrapped around a filter unit, with electric heating inside of theinsulating layer to maintain the walls of the filter unit at a desiredelevated temperature higher than the temperature of filter elements inthe filter unit, thereby reducing thermophoretic particle losses towalls of the filter unit.

Even with careful operation to reduce thermophoretic losses, some losseswill still occur. For example, some particles will inevitably be lost towalls of particle collection equipment, such as the walls of a cycloneor filter housing. One way to reduce these losses, and correspondinglyincrease product yield, is to periodically wash the interior of theparticle collection equipment to remove particles adhering to the sides.In most cases, the wash fluid will be water, unless water would have adetrimental effect on one of the components of the particles. Forexample, the particle collection equipment could include parallelcollection paths. One path could be used for active particle collectionwhile the other is being washed. The wash could include an automatic ormanual flush without disconnecting the equipment. Alternatively, theequipment to be washed could be disconnected to permit access to theinterior of the equipment for a thorough wash. As an alternative tohaving parallel collection paths, the process could simply be shut downoccasionally to permit disconnection of the equipment for washing. Theremoved equipment could be replaced with a clean piece of equipment andthe process could then be resumed while the disconnected equipment isbeing washed.

For example, a cyclone or filter unit could periodically be disconnectedand particles adhering to interior walls could be removed by a waterwash. The particles could then be dried in a low temperature dryer,typically at a temperature of lower than about 50° C.

In one embodiment, wash fluid used to wash particles from the interiorwalls of particle collection equipment includes a surfactant. Some ofthe surfactant will adhere to the surface of the particles. This couldbe advantageous to reduce agglomeration tendency of the particles and toenhance dispersibility of the particles in a thick film pastformulation. The surfactant could be selected for compatibility with thespecific paste formulation anticipated.

Another area for potential losses in the system, and for the occurrenceof potential operating problems, is between the outlet of the aerosolgenerator and the inlet of the furnace. Losses here are not due tothermophoresis, but rather to liquid coming out of the aerosol andimpinging and collecting on conduit and equipment surfaces. Althoughthis loss is undesirable from a material yield standpoint, the loss maybe even more detrimental to other aspects of the process. For example,water collecting on surfaces may release large droplets that can lead tolarge particles that detrimentally contaminate the particulate product.Furthermore, if accumulated liquid reaches the furnace, the liquid cancause excessive temperature gradients within the furnace tube, which cancause furnace tube failure, especially for ceramic tubes.

One way to reduce the potential for undesirable liquid buildup in thesystem is to provide adequate drains, as previously described. In thatregard, it is preferred that a drain be placed as close as possible tothe furnace inlet to prevent liquid accumulations from reaching thefurnace. The drain should be placed, however, far enough in advance ofthe furnace inlet such that the stream temperature is lower than about80° C. at the drain location.

Another way to reduce the potential for undesirable liquid buildup isfor the conduit between the aerosol generator outlet and the furnaceinlet be of a substantially constant cross sectional area andconfiguration. Preferably, the conduit beginning with the aerosolgenerator outlet, passing through the furnace and continuing to at leastthe cooling unit inlet is of a substantially constant cross sectionalarea and geometry.

Another way to reduce the potential for undesirable buildup is to heatat least a portion, and preferably the entire length, of the conduitbetween the aerosol generator and the inlet to the furnace. For example,the conduit could be wrapped with a heating tape to maintain the insidewalls of the conduit at a temperature higher than the temperature of theaerosol. The aerosol would then tend to concentrate toward the center ofthe conduit due to thermophoresis. Fewer aerosol droplets would,therefore, be likely to impinge on conduit walls or other surfacesmaking the transition to the furnace.

Another way to reduce the potential for undesirable liquid buildup is tointroduce a dry gas into the aerosol between the aerosol generator andthe furnace. Referring now to FIG. 49, one embodiment of the process isshown for adding a dry gas 118 to the aerosol 108 before the furnace110. Addition of the dry gas 118 causes vaporization of at least a partof the moisture in the aerosol 108, and preferably substantially all ofthe moisture in the aerosol 108, to form a dried aerosol 119, which isthen introduced into the furnace 110.

The dry gas 118 will most often be dry air, although in some instancesit may be desirable to use dry nitrogen gas or some other dry gas. Ifsufficient a sufficient quantity of the dry gas 118 is used, thedroplets of the aerosol 108 are substantially completely dried tobeneficially form dried precursor particles in aerosol form forintroduction into the furnace 110, where the precursor particles arethen pyrolyzed to make a desired particulate product. Also, the use ofthe dry gas 118 typically will reduce the potential for contact betweendroplets of the aerosol and the conduit wall, especially in the criticalarea in the vicinity of the inlet to the furnace 110. In that regard, apreferred method for introducing the dry gas 118 into the aerosol 108 isfrom a radial direction into the aerosol 108. For example, equipment ofsubstantially the same design as the quench cooler, described previouslywith reference to FIGS. 41–43, could be used, with the aerosol 108flowing through the interior flow path of the apparatus and the dry gas118 being introduced through perforated wall of the perforated conduit.An alternative to using the dry gas 118 to dry the aerosol 108 would beto use a low temperature thermal preheater/dryer prior to the furnace110 to dry the aerosol 108 prior to introduction into the furnace 110.This alternative is not, however, preferred.

Still another way to reduce the potential for losses due to liquidaccumulation is to operate the process with equipment configurationssuch that the aerosol stream flows in a vertical direction from theaerosol generator to and through the furnace. For smaller-sizeparticles, those smaller than about 1.5 μm, this vertical flow should,preferably, be vertically upward. For larger-size particles, such asthose larger than about 1.5 μm, the vertical flow is preferablyvertically downward.

Furthermore, with the process of the present invention, the potentialfor system losses is significantly reduced because the total systemretention time from the outlet of the generator until collection of theparticles is typically shorter than about 10 seconds, preferably shorterthan about 7 seconds, more preferably shorter than about 5 seconds andmost preferably shorter than about 3 seconds.

For the production of oxygen-containing phosphors according to thepresent invention, the liquid feed includes the chemical components thatwill form the phosphor particles. For example, the liquid feed caninclude a solution containing nitrates, chlorides, sulfates, hydroxidesor oxalates of the phosphor compound. A preferred precursor are thenitrates, such as yttrium nitrate, Y(NO₃)₃.6H₂O, for the production ofyttria phosphor particles. Nitrates are typically highly soluble inwater and the solutions maintain a low viscosity, even at highconcentrations. A typical reaction mechanism would be:2Y(NO₃)₃+H₂O+heat→Y₂O₃+NO_(x)+H₂O

The solution is preferably not saturated with the precursor to avoidprecipitate formation in the liquid. The solution preferably includes,for example, sufficient precursor to yield from about 1 to 50 weightpercent, such as from about 1 to 15 weight percent, of the phosphorcompound, based on the amount of metals in solution. The final particlesize of the phosphor particles is also influenced by the precursorconcentration. Generally, lower precursor concentrations in the liquidfeed will produce particles having a smaller average size.

In addition to the host material, the liquid feed preferably includesthe precursor to the activator ion. For example, for the production ofY₂O₃:Eu phosphor particles, the precursor solution preferably includesyttrium nitrate, as is discussed above, and also europium nitrate. Therelative concentrations of the precursors can be adjusted to vary theconcentration of the activator ion in the host material.

Preferably, the solvent is aqueous-based for ease of operation, althoughother solvents, such as toluene, may be desirable. The use of organicsolvents can lead to undesirable carbon contamination in the phosphorparticles. The pH of the aqueous-based solutions can be adjusted toalter the solubility characteristics of the precursor in the solution.

In addition to the foregoing, the liquid feed may also include otheradditives that contribute to the formation of the particles. Forexample, a fluxing agent can be added to the solution to increase thecrystallinity and/or density of the particles. For example, the additionof urea to metal salt solutions, such as a metal nitrate, can increasethe density of particles produced from the solution. In one embodiment,up to about 1 mole equivalent urea is added to the precursor solution,as measured against the moles of phosphor compound in the metal saltsolution. Further, if the particles are to be coated phosphor particles,as is discussed in more detail below, a soluble precursor to both theoxygen-containing phosphor compound and the coating can be used in theprecursor solution wherein the coating precursor is an involatile orvolatile species.

For producing oxygen-containing phosphor particles, the carrier gas maycomprise any gaseous medium in which droplets produced from the liquidfeed may be dispersed in aerosol form. Also, the carrier gas may beinert, in that the carrier gas does not participate in formation of thephosphor particles. Alternatively, the carrier gas may have one or moreactive component(s) that contribute to formation of the phosphorparticles. In that regard, the carrier gas may include one or morereactive components that react in the furnace to contribute to formationof the phosphor particles. In many applications for the production ofoxygen-containing phosphor particles, air will be a satisfactory carriergas. In other instances, a relatively inert gas such as nitrogen may berequired.

When the oxygen-containing phosphors are coated phosphors, precursors tometal oxide coatings can be selected from volatile metal acetates,chlorides, alkoxides or halides. Such precursors are known to react athigh temperatures to form the corresponding metal oxides and eliminatesupporting ligands or ions. For example, SiCl₄ can be used as aprecursor to SiO₂ coatings when water vapor is present:SiCl_(4(g))+2H₂O_((g))→SiO_(2(s))+4HCl_((g))SiCl₄ also is highly volatile and is a liquid at room temperature, whichmakes transport into the reactor more controllable.

Metal alkoxides can be used to produce metal oxide films by hydrolysis.The water molecules react with the alkoxide M-O bond resulting in cleanelimination of the corresponding alcohol with the formation of M-O-Mbonds:Si(OEt)₄+2H₂O→SiO₂+4EtOHMost metal alkoxides have a reasonably high vapor pressure and aretherefore well suited as coating precursors.

Metal acetates are also useful as coating precursors since they readilydecompose upon thermal activation by acetic anhydride elimination:Mg(O₂CCH₃)₂→MgO+CH₃C(O)OC(O)CH₃Metal acetates are advantageous as coating precursors since they arewater stable and are reasonably inexpensive.

Coatings can be generated on the particle surface by a number ofdifferent mechanisms. One or more precursors can vaporize and fuse tothe hot phosphor particle surface and thermally react resulting in theformation of a thin-film coating by chemical vapor deposition (CVD).Preferred coatings deposited by CVD include metal oxides and elementalmetals. Further, the coating can be formed by physical vapor deposition(PVD) wherein a coating material physically deposits an the surface ofthe particles. Preferred coatings deposited by PVD include organicmaterials and elemental metal. Alternatively, the gaseous precursor canreact in the gas phase forming small particles, for example less thanabout 5 nanometers in size, which then diffuse to the larger particlesurface and sinter onto the surface, thus forming a coating. This methodis referred to as gas-to-particle conversion (GPC). Whether such coatingreactions occur by CVD, PVD or GPC is dependent on the reactorconditions such as precursor partial pressure, water partial pressureand the concentration of particles in the gas stream. Another possiblesurface coating method is surface conversion of the surface of theparticle by reaction with a vapor phase reactant to convert the surfaceof the particles to a different material than that originally containedin the particles.

In addition, a volatile coating material such as PbO, MoO₃ or V₂O₅ canbe introduced into the reactor such that the coating deposits on theparticle by condensation. Highly volatile metals, such as silver, canalso be deposited by condensation. Further, the phosphor powders can becoated using other techniques. For example, a soluble precursor to boththe phosphor powder and the coating can be used in the precursorsolution wherein the coating precursor is involatile (e.g. Al(NO₃)₃) orvolatile (e.g. Sn(OAc)₄ where Ac is acetate). In another method, acolloidal precursor and a soluble phosphor precursor can be used to forma particulate colloidal coating on the phosphor.

The structural modification that occurs in the particle modifier may beany modification to the crystalline structure or morphology of theparticles. For example, the particles can be annealed in the particlemodifier to densify the particles or to recrystallize the particles intoa polycrystalline or single crystalline form. Also, especially in thecase of composite particles, the particles may be annealed for asufficient time to permit redistribution within the particles ofdifferent material phases or permit redistribution of the activatorion(s).

More specifically, while the oxygen-containing phosphor powders producedby the foregoing method have good crystallinity, it may be desirable toincrease the crystallinity (average crystallite size) after production.Thus, the powders can be annealed (heated) for an amount of time and ina preselected environments to increase the crystallinity of the phosphorparticles. Increased crystallinity can advantageously yield an increasedbrightness and efficiency of the phosphor particles. If such annealingsteps are performed, the annealing temperature and time should beselected to minimize the amount of interparticle sintering that is oftenassociated with annealing. According to one embodiment of the presentinvention, the oxygen-containing phosphor powder is preferably annealedat a temperature of from about 700° C. to about 1700° C., morepreferably from about 1100° C. to about 1400° C. The annealing time ispreferably not more than about 2 hours and can be as little as about 1minute. The oxygen-containing powders are typically annealed in an inertgas, such as argon or in an oxygen-containing gas such air.

Further, the crystallinity of the phosphors can be increased by using afluxing agent, either in the precursor solution or in a post-formationannealing step. A fluxing agent is a reagent which improves thecrystallinity of the material when the reagent and the material areheated together, as compared to heating the material to the sametemperature and for the same amount of time in the absence of thefluxing agent. The fluxing agents typically cause a eutectic to formwhich leads to a liquid phase at the grain boundaries, increasing thediffusion coefficient. The fluxing agent, for example alkali metalhalides such as NaCl or KCl or an organic compound such as urea(CO(NH₂)₂), can be added to the precursor solution where it improves thecrystallinity and/or density of the particles during their subsequentformation. Alternatively, the fluxing agent can be contacted with thephosphor powder batches after they have been collected. Upon annealing,the fluxing agent improves the crystallinity of the phosphor powder, andtherefore improves other properties such as the brightness of thephosphor powder. Also, in the case of composite particles 112, theparticles may be annealed for a sufficient time to permit redistributionwithin the particles 112 of different material phases.

The present invention is particularly applicable to oxygen-containingphosphors. Phosphors are materials which are capable of emittingradiation in the visible or ultraviolet spectral range upon excitation,such as excitation by an external electric field or other externalenergy source. Oxygen-containing phosphors are those phosphors that havea host material that includes oxygen, such as a host material based on ametal oxide, a silicate, borate or aluminate. Examples of suchoxygen-containing phosphors are given in more detail below Theoxygen-containing phosphors can be chemically tailored to emit specificwavelengths of visible light, such as red, blue or green light. Bydispersing various phosphor powders in a predetermined arrangement andcontrollably exciting the powders, a full-color visual display can beachieved.

Oxygen-containing phosphors typically include a matrix compound,referred to as a host material, and the phosphor further includes one ormore dopants, referred to as activator ions, to emit a specific color orto enhance the luminescence characteristics. Some phosphors, such asup-converter phosphors, incorporate more than one activator ion.

Phosphors can be classified by their phosphorescent properties and thepresent invention is applicable to all types of these phosphors. Forexample, electroluminescent phosphors are phosphors that emit light uponstimulation by an electric field. These phosphors are used for thin-filmand thick-film electroluminescent displays, back lighting for LCD's andelectroluminescent lamps used in wrist watches and the like.Cathodoluminescent phosphors emit light upon stimulation by electronbombardment. These phosphors are utilized in CRT's (e.g. commontelevisions) and FED's.

Photoluminescent phosphors emit light upon stimulation by other light.The stimulating light usually has higher energy than the emitted light.For example, a photoluminescent phosphor can emit visible light whenstimulated by ultraviolet light. These phosphors are utilized in plasmadisplay panels and common fluorescent lamps.

Up-converter phosphors also emit light upon stimulation by other light,but usually light of a lower energy than the emitted light. For example,infrared light can be used to stimulate an up-converter phosphor whichthen emits visible or ultraviolet light. Up-convertor phosphorstypically include at least 2 activator ions which convert the lowerenergy infrared light. These materials are useful in immunoassay andsecurity applications. Similarly, x-ray phosphors are utilized toconvert x-rays to visible light and are useful in medical diagnostics.

The oxygen-containing host material can be doped with an activator ion,such as in an amount of from about 0.02 to about 15 atomic percent,preferably from about 0.02 to about 10 atomic percent and morepreferably from about 0.02 to about 5 atomic percent. It will beappreciated, as is discussed in more detail below, that the preferredconcentration of activator ion in the host material can vary fordifferent applications. In the case of the phosphor compound ZnO, themetal oxide is produced to be slightly off-stoichiometric such that Znis the activator ions.

One advantage of the present invention is that the activator ion ishomogeneously distributed throughout the host material. Phosphor powdersprepared by solid-state methods do not give uniform concentration of theactivator ion in small particles and solution routes also do not givehomogenous distribution of the activator ion due to different rates ofprecipitation.

Particular phosphor compounds may be preferred for certain applicationsand no single phosphor compound is necessarily preferred for allpossible applications. As used herein, oxygen-containing phosphorcompounds are those that include a host material selected from simple orcomplex metal oxides, metal silicates, metal borates, or titanates.

Examples of metal oxide phosphor compounds include, but are not limitedto, Y₂O₃:Eu, ZnO:Zn, Y₃Al₅O₁₂:Tb and barium aluminates, such asBaMgAl₁₄O₂₃:Eu.

Examples of metal silicate phosphors include silicates such asZn₂SiO₄:Mn, Ca₂SiO₄:Eu, Ba₂SiO₄:Eu, Gd₂SiO₅:Ce and Y₂SiO₅:Ce. Examplesof metal borates include (Y,Gd)BO₃:Eu. An example of a titanate isCaTiO₃:RE, where RE is a rare-earth element.

Particularly preferred oxygen-containing phosphor host materials forsome display applications include ZnO:Zn and Y₂O₃:Eu. Further examplesof preferred oxygen-containing phosphor host materials and activatorions are listed in Table 1.

TABLE I Examples of Oxygen-containing Phosphor Materials Host MaterialActivator Ion Color ZnO Zn Green Y₂O₃ Eu Red BaMgAl₁₄O₂₃ Eu BlueY₃Al₅O₁₂ Tb Green Zn₂SiO₄ Mn Green Ca₂SiO₄ Eu Green Ba₂SiO₄ Eu GreenY₂SiO₅ Ce Blue (Y, Gd) BO₃ Eu Red

Certain of the foregoing oxygen-containing phosphor compounds aredifficult to produce using conventional methods such that the powdershave the desirable luminescent characteristics. Examples include ternarymetal oxides such as YAG (Y₃Al₅O₁₂) and BAM (BaMgAl₁₄O₂₃), silicatessuch as Zn₂SiO₄ and Y₂SiO₄ and borates such as (Y,Gd)BO₃. Such compoundscan be difficult to produce even using a standard spray pyrolysistechniques and typically require post-treatment to achieve the desiredlevel of crystallinity.

These compounds can advantageously be produced according to the presentinvention using a process referred to herein as spray-conversion.Spray-conversion is a process wherein a spray pyrolysis technique, as isdescribed in detail previously, is used to produce an intermediateproduct, such as an oxide, that is capable of being subsequentlyconverted to the oxygen-containing phosphor. The intermediate productadvantageously has many of the desirable morphological and chemicalproperties discussed hereinbelow, such as a small particle size and highpurity.

Water-soluble precursor materials, such as nitrate salts, are placedinto solution and are converted at a low temperature, such as less thanabout 1000° C., to an admixture of oxide phases. The oxide phase is inthe form of small particles having a narrow size distribution, as isdescribed in more detail below. The intermediate product is thenconverted by further treatment, such as by annealing in anoxygen-containing atmosphere at an elevated temperature to form asubstantially phase pure phosphor compound having high crystallinity.The resulting powder can be gently milled to remove any softagglomerates that result from the annealing process. The powder can alsobe annealed to increase the crystallinity of the powders. The resultingend product is an oxygen-containing phosphor powder having the desirablemorphological and luminescent properties. The average particle size andmorphological characteristics are determined by the characteristics ofthe intermediate product. Although discussed herein with reference tospecific phosphor compounds, it will be appreciated that other phosphorscan be produced using a similar spray-conversion process. Thus, theprecursors, such as nitrate salts, can be spray-converted at atemperature of, for example, less than about 1000° C. to form ahomogeneous admixture of one or more oxides having low crystallinity.The oxides can then be annealed at a temperature of, for example, 800°C. to 1700° C., to form the oxygen-containing phosphor compounds. Thephosphor particles can be lightly milled to remove agglomerates and canbe further annealed to increase crystallinity of the particles, possiblyin the presence of a fluxing agent.

The powder characteristics that are preferred will depend upon theapplication of the oxygen-containing phosphor powders. Nonetheless, itcan be generally stated that the powders should usually have a smallparticle size, narrow size distribution, spherical morphology, highdensity/low porosity, high crystallinity and homogenous dopantdistribution of activator ion throughout the host material. Theefficiency of the phosphor, defined as the overall conversion rate ofexcitation energy to visible photons, should be high.

According to the present invention, the oxygen-containing phosphorpowder includes particles having a small average size. Although thepreferred average size of the phosphor particles will vary according tothe application of the phosphor powder, the average particle size of thephosphor particles is not greater than about 10 μm. For mostapplications, the average particle size is preferably not greater thanabout 5 μm, such as from about 0.1 μm to about 5 μm and more preferablyis not greater than about 3 μm, such as from about 0.3 μm to about 3 μm.As used herein, the average particle size is the weight average particlesize.

According to the present invention, the powder batch of phosphorparticles also has a narrow particle size distribution, such that themajority of particles are substantially the same size. Preferably, atleast about 90 weight percent of the particles and more preferably atleast about 95 weight percent of the particles are not larger than twicethe average particle size. Thus, when the average particle size is about2 μm, it is preferred that at least about 90 weight percent of theparticles are not larger than 4 μm and it is more preferred that atleast about 95 weight percent of the particles are not larger than 4 μm.Further, it is preferred that at least about 90 weight percent of theparticles, and more preferably at least about 95 weight percent of theparticles, are not larger than about 1.5 times the average particlesize. Thus, when the average particle size is about 2 μm, it ispreferred that at least about 90 weight percent of the particles are notlarger than about 3 μm and it is more preferred that at least about 95weight percent of the particles are not larger than about 3 μm.

The phosphor particles of the present invention can be substantiallysingle crystal particles or may be comprised of a number ofcrystallites. According to the present invention, the phosphor particlesare highly crystalline and it is preferred that the average crystallitesize approaches the average particle size such that the particles aremostly single crystals or are composed of only a few large crystals. Theaverage crystallite size of the particles is preferably at least about25 nanometers, more preferably is at least about 40 nanometers, evenmore preferably is at least about 60 nanometers and most preferably isat least about 80 nanometers. In one embodiment, the average crystallitesize is at least about 100 nanometers. As it relates to particle size,the average crystallite size is preferably at least about 20 percent,more preferably at least about 30 percent and most preferably is atleast about 40 percent of the average particle size. Such highlycrystalline phosphors are believed to have increased luminescentefficiency and brightness as compared to phosphor particles havingsmaller crystallites.

The oxygen-containing phosphor particles of the present inventionadvantageously have a high degree of purity, that is, a low level ofimpurities. Impurities are those materials that are not intended in thefinal product. Thus, an activator ion is not considered an impurity. Thelevel of impurities in the phosphor powders of the present invention ispreferably not greater than about 1 atomic percent, more preferably isnot greater than about 0.1 atomic percent and even more preferably isnot greater than about 0.01 atomic percent.

The oxygen-containing phosphor particles are also very dense (notporous) as measured by helium pychnometry. Preferably, the particleshave a particle density of at least about 80 percent of the theoreticaldensity for the host material, more preferably at least about 90 percentof the theoretical density for the host material and even morepreferably at least about 95 percent of the theoretical density for thehost material.

The oxygen-containing phosphor particles of the present invention arealso substantially spherical in shape. That is, the particles are notjagged or irregular in shape. Spherical particles are particularlyadvantageous because they are able to disperse and coat a device, suchas a display panel, more uniformly with a reduced average thicknessAlthough the particles are substantially spherical, the particles maybecome faceted as the crystallite size increases and approaches theaverage particle size.

In addition, the oxygen-containing phosphor particles according to thepresent invention advantageously have a low surface area. The particlesare substantially spherical, which reduces the total surface area for agiven mass of powder. Further, the elimination of larger particles fromthe powder batches eliminates the porosity that is associated with openpores on the surface of such larger particles. Due to the elimination ofthe large particles, the powder advantageously has a lower surface area.Surface area is typically measured using a BET nitrogen adsorptionmethod which is indicative of the surface area of the powder, includingthe surface area of accessible surface pores on the surface of thepowder. For a given particle size distribution, a lower value of asurface area per unit mass of powder indicates solid or non-porousparticles. Decreased surface area reduces the susceptibility of thephosphor powders to adverse surface reactions, such as degradation frommoisture. This characteristic can advantageously extend the useful lifeof the phosphor powders.

The surfaces of the oxygen-containing phosphor particles according tothe present invention are typically smooth and clean with a minimaldeposition of contaminants on the particle surface. For example, theouter surfaces are not contaminated with surfactants, as is often thecase with particles produced by liquid precipitation routes.

In addition, the powder batches of oxygen-containing phosphor particlesaccording to the present invention are substantially unagglomerated,that is, they include substantially no hard agglomerates or particles.Hard agglomerates are physically coalesced lumps of two or moreparticles that behave as one large particle. Agglomerates aredisadvantageous in most applications of phosphor powders. It ispreferred that no more than about 1 weight percent of the phosphorparticles in the powder batch of the present invention are in the formof hard agglomerates. More preferably, no more than about 0.5 weightpercent of the particles are in the form of hard agglomerates and evenmore preferably no more than about 0.1 weight percent of the particlesare in the form of hard agglomerates.

According to one embodiment of the present invention, theoxygen-containing phosphor particles are composite phosphor particles,wherein the individual particles include at least one oxygen-containingphosphor phase and at least a second phase associated with the phosphorphase. The second phase can be a different phosphor compound or can be anon-phosphor compound. Such composites can advantageously permit the useof phosphor compounds in devices that would otherwise be unusable.Further, combinations of different phosphor compounds within oneparticle can produce emission of a selected color. The emission of thetwo phosphor compounds would combine to approximate white light.Further, in cathodoluminescent applications, the matrix material canaccelerate the impingent electrons to enhance the luminescence.

According to another embodiment of the present invention, the phosphorparticles are surface modified or coated phosphor particles that includea particulate coating (FIG. 47 d) for non-particulate (film) coating(FIG. 47 a) that substantially encapsulates an outer surface of theparticles. The coating can be a metal, a non-metallic compound or anorganic compound.

Coatings are often desirable to reduce degradation of theoxygen-containing phosphor material due to moisture or other influences,such as the plasma in a plasma display device or high density electronbombardment in cathodoluminescent devices. The thin, uniform coatingsaccording to the present invention will advantageously permit use of thephosphor powders under low voltage, high current conditions. Coatingsalso create a diffusion barrier such that activator ions (e.g. Cu andMn) cannot transfer from one particle to another, thereby altering theluminescence characteristics. Coatings can also control the surfaceenergy levels of the particles.

The coating can be a metal, metal oxide or other inorganic compound suchas a metal sulfide, or can be an organic compound. For example, a metaloxide coating can advantageously be used, such as a metal oxide selectedfrom the group consisting of SiO₂, MgO, Al₂O₃, ZnO, SnO₂ or In₂O₃.Particularly preferred are SiO₂ and Al₂O₃ coatings. Semiconductive oxidecoatings such as SnO₂ or In₂O₃ can be advantageous in some applicationsdue to the ability of the coating to absorb secondary electrons that areemitted by the phosphor. Metal coatings, such as copper, can be usefulfor phosphor particles used in direct current electroluminescentapplications. In addition, phosphate coatings, such as zirconiumphosphate or aluminum phosphate, can also be advantageous for use insome applications.

The coatings should be relatively thin and uniform. The coating shouldencapsulate the entire particle, but be sufficiently thin such that thecoating doesn't interfere with light transmission. Preferably, thecoating has an average thickness of not greater than about 200nanometers, more preferably not greater than about 100 nanometers, andeven more preferably not greater than about 50 nanometers. The coatingpreferably completely encapsulates the phosphor particle and thereforeshould have an average thickness of at least about 2 nanometers, morepreferably at least about 5 nanometers. In one embodiment, the coatinghas a thickness of from about 2 to 50 nanometers, such as from about 2to 10 nanometers. Further, the particles can include more than onecoating substantially encapsulating the particles to achieve the desiredproperties.

The coating, either particulate or non-particulate, can also include apigment or other material that alters the light characteristics of thephosphor. Red pigments can include compounds such as the iron oxides(Fe₂O₃), cadmium sulfide compounds (CdS) or mercury sulfide compounds(HgS). Green or blue pigments include cobalt oxide (CoO), cobaltaluminate (CoAl₂O₄) or zinc oxide (ZnO). Pigment coatings are capable ofabsorbing selected wavelengths of light leaving the phosphor, therebyacting as a filter to improve the color contrast and purity,particularly in CRT devices.

In addition, the phosphor particles can be coated with an organiccompound such as PMMA (polymethylmethacrylate), polystyrene or similarorganic compounds, including surfactants that aid in the dispersionand/or suspension of the particles in a flowable medium. The organiccoating is preferably not greater than about 100 nanometers thick and issubstantially dense and continuous about particle. The organic coatingscan advantageously prevent corrosion of the phosphor particles,especially in electroluminescent lamps, and also can improve thedispersion characteristics of the particles in a paste or other flowablemedium.

The coating can also be comprised of one or more monolayer coatings,such as from about 1 to 3 monolayer coatings. A monolayer coating isformed by the reaction of an organic or an inorganic molecule with thesurface of the phosphor particles to form a coating layer that isessentially one molecular layer thick. In particular, the formation of amonolayer coating by reaction of the surface of the phosphor powder witha functionalized organo silane such as halo- or amino-silanes, forexample hexamethyldisilazane or trimethylsilylchloride, can be used tomodify and control the hydrophobicity and hydrophilicity of the phosphorpowders. Monolayer coatings of metal oxides (e.g. ZnO or SiO₂) or metalsulfides (e.g. Cu₂S) can be formed as monolayer coatings. Monolayercoatings can allow for greater control over the dispersioncharacteristics of the phosphor powder in a wide variety of pastecompositions and other flowable mediums.

The monolayer coatings may also be applied to phosphor powders that havealready been coated with an organic or inorganic coating, thus providingbetter control over the corrosion characteristics (through the use of athicker coating) as well as dispersibility (through the use of amonolayer coating) of the phosphor powder.

As a direct result of the foregoing powder characteristics, theoxygen-containing phosphor powders of the present invention have manyunique and advantageous properties that are not found in phosphorpowders known heretofore.

The oxygen-containing phosphor powders of the present invention have ahigh efficiency, sometimes referred to as quantum efficiency. Efficiencyis the overall conversion rate of excitation energy (electrons orphotons) to visible photons emitted. According to one embodiment of thepresent invention, the efficiency of the phosphor powder is at leastabout 90%. The near perfect efficiency of the phosphor powders accordingto the present invention is believed to be due to the high crystallinityand homogenous distribution of activator ion in the host material.

The oxygen-containing phosphor powders also have well-controlled colorcharacteristics, sometimes referred to as emission spectrumcharacteristics or chromaticity. This important property is due to theability to precisely control the composition of the host material, thehomogenous distribution of the activator ion and the high purity of thepowders.

The phosphor powders also have improved decay time, also referred to aspersistence Persistence is referred to as the amount of time for thelight emission to decay to 10% of its brightness. Phosphors with longdecay times can result in blurred images when the image moves across thedisplay. The improved decay time of the phosphor powders of the presentinvention is believed to be due primarily to the homogenous distributionof activator ion in the host material.

The phosphor powders also have an improved brightness over prior artphosphor powders. That is, under a given application of energy, thephosphor powders of the present invention produce more light.

Thus, the oxygen-containing phosphor powders of the present inventionhave a unique combination of properties that are not found inconventional phosphor powders. The powders can advantageously be used toform a number of intermediate products, for example pastes or slurries,and can be incorporated into a number of devices, wherein the deviceswill have significantly improved performance resulting directly from thecharacteristics of the phosphor powders of the present invention. Thedevices can include light-emitting lamps and display devices forvisually conveying information and graphics. Such display devicesinclude traditional CRT-based display devices, such as televisions, andalso include flat panel displays. Flat panel displays are relativelythin devices that present graphics and images without the use of atraditional picture tube and operate with modest power requirements.Generally, flat panel displays include a phosphor powder selectivelydispersed on a viewing panel, wherein the excitation source lies behindand in close proximity to the panel. Flat panel displays include liquidcrystal displays (LCD), plasma display panels (PDP's) electroluminescent(EL) displays, and field emission displays (FED'S).

CRT devices, utilizing a cathode ray tube, include traditional displaydevices such as televisions and computer monitors. CRT's operate byselectively firing electrons from one or more cathode ray tubes atcathodoluminescent phosphor particles which are located in predeterminedregions (pixels) of a display screen. The cathode ray tube is located ata distance from the display screen which increases as screen sizeincreases, By selectively directing the electron beam at certain pixels,a full color display with high resolution can be achieved.

A CRT display device is illustrated schematically in FIG. 50. The device1002 includes 3 cathode ray tubes 1004, 1006 and 1008 located in therear portion of the device. The cathode ray tubes generate electrons,such as electron 1010. An applied voltage of 20 to 30 kV accelerates theelectrons toward the display screen 1012. In a color CRT, the displayscreen is patterned with red (R), green (G) and blue (B) phosphors, asis illustrated in FIG. 51. Three colored phosphor pixels are grouped inclose proximity, such as group 1014, to produce multicolor images.Graphic output is created be selectively directing the electrons at thepixels on the display screen 1012 using, for example, electromagnets1016. The electron beams are rastered in a left to right, top to bottomfashion to create a moving image. The electrons can also be filteredthrough an apertured metal mask to block electrons that are directed atthe wrong phosphor.

The phosphor powder is typically applied to the CRT display screen usinga slurry. The slurry is formed by suspending the phosphor particles inan aqueous solution which can also include additives such as PVA(polyvinyl alcohol) and other organic compounds to aid in the dispersionof the particles in the solution as well as other compounds such asmetal chromates. The display screen is placed in a coating machine, suchas a spin coater, and the slurry is deposited onto the inner surface ofthe display screen and spread over the entire surface. The displayscreen is spun to thoroughly coat the surface and spin away any excessslurry. The slurry on the screen is then dried and exposed through ashadow mask having a predetermined dot-like or stripe-like pattern. Theexposed film is developed and excess phosphor particles are washed awayto form a phosphor screen having a predetermined pixel pattern. Theprocess can be performed in sequence for different color phosphors toenable a full color display to be produced.

It is generally desired that the pixels are formed with a highly uniformphosphor powder layer thickness. The phosphors should not peel from thedisplay screen and no cross contamination of the colored phosphorsshould occur. These characteristics are significantly influenced by themorphology, size and surface condition of the phosphor particles.

CRT devices typically employ phosphor particles rather than thin-filmphosphors due to the high luminescence requirements. The resolution ofimages on powdered phosphor screens can be improved if the screen ismade with particles having a small size and uniform size distributionsuch as the phosphor particles according to the present invention. Imagequality on the CRT device is also influenced by the packing voids of theparticles and the number of layers of phosphor particles which are notinvolved in the generation of cathodoluminescence. That is, particleswhich are not excited by the electron beam will only inhibit thetransmission of luminescence through the device. Large particles andaggregated particles both form voids and further contribute to loss oflight transmission. Significant amounts of light can be scattered byreflection in voids. Further, for a high quality image, the phosphorlayer should have a thin and highly uniform thickness. Ideally, theaverage thickness of the phosphor layer should be about 1.5 times theaverage particle size of the phosphor particles.

CRT's typically operate at high voltages such as from about 20 kV to 30kV. Phosphors used for CRT's should have high brightness and goodchromaticity. Oxygen-containing phosphors useful for CRT's includeY₃Al₃O₁₂:Tb for green. The phosphor particles can advantageously becoated in accordance with the present invention to prevent degradationof the host material or diffusion of activator ions. Silica or silicatecoatings can also improve the rheological properties of the phosphorslurry. The particles can also include a pigment coating, such asparticulate Fe₂O₃, to modify and enhance the properties of the emittedlight.

The introduction of high-definition televisions (HDTV) has increased theinterest in projection television (PTV). In this concept, the lightproduced by three independent cathode ray tubes is projected onto afaceplate on the tube that includes particulate phosphors, to form 3colored projection images. The three images are projected onto a displayscreen by reflection to produce a full color image. Because of the largemagnification used in imaging, the phosphors on the faceplate of thecathode ray tube must be excited with an intense and small electronspot. Maximum excitation density may be two orders of magnitude largerthan with conventional cathode ray tubes. Typically, the efficiency ofthe phosphor decreases with increasing excitation density. For suchapplications, the red phosphor Y₂O₃:Eu is preferred since it shows lesssaturation than, for example, an oxysulfide. Also, the temperaturedependence of the oxide is less than the oxysulfide which is importantsince the temperature of the phosphor in a PTV tube may increase up to100° C. For the foregoing reasons, the metal-oxide phosphor powders ofthe present invention having a small size and narrow size distributionare particularly useful in HDTV applications.

One of the problems with CRT-based devices is that they are large andbulky and have significant depth as compared to the screen size.Therefore, there is significant interest in developing flat paneldisplays to replace CRT-based devices in many applications.

Flat panel displays (FPD's) offer many advantages over CRT's includinglighter weight, portability and decreased power requirements. Flat paneldisplays can be either monochrome or color displays. It is believed thatflat panel displays will eventually replace the bulky CRT devices, suchas televisions, with a thin product that can be hung on a wall, like apicture. Currently, flat panel displays can be made thinner, lighter andwith lower power consumption than CRT devices, but not with the visualquality and cost performance of a CRT device.

The high electron voltages and small currents traditionally required toactivate phosphors efficiently in a CRT device have hindered thedevelopment of flat panel displays. Phosphors for flat panel displayssuch as field emission displays must typically operate at a lowervoltage, higher current density and higher efficiency than phosphorsused in existing CRT devices. The low voltages used in such displaysresult in an electron penetration depth in the range of severalmicrometers down to tens of nanometers, depending on the appliedvoltage. Thus, the control of the size and crystallinity of the phosphorparticles is critical to device performance. If large or agglomeratedpowders are used, only a small fraction of the electrons will interactwith the phosphor. Use of phosphor powders having a wide sizedistribution can also lead to non-uniform pixels and sub-pixels, whichwill produce a blurred image.

One type of FPD is a plasma display panel (PDP). Plasma displays haveimage quality that is comparable to current CRT devices and can beeasily scaled to large sizes such as 20 to 60 diagonal inches. Thedisplays are bright and lightweight and have a thickness of from about1.5 to 3 inches. A plasma display functions in a similar manner asfluorescent lighting. In a plasma display, plasma source, typically agas mixture, is placed between an opposed array of addressableelectrodes and a high energy electric field is generated between theelectrodes. Upon reaching a critical voltage, a plasma is formed fromthe gas and UV photons are emitted by the plasma. Color plasma displayscontain three-color photoluminescent phosphor particles deposited on theinside of the glass faceplate. The phosphors selectively emit light whenilluminated by the photons. Plasma displays operate at relatively lowcurrents and can be driven either by an AC or DC signal. AC plasmasystems use a dielectric layer over the electrode, which forms acapacitor. This impedance limits current and provides a necessary chargein the gas mixture.

A cross-section of a plasma display device is illustrated in FIG. 52.The plasma display 1040 comprises two opposed panels 1042 and 1044 inparallel opposed relation. A working gas is disposed and sealed betweenthe two opposing panels 1042 and 1044. The rear panel 1044 includes abacking plate 1046 on which are printed a plurality of electrodes 1048(cathodes) which are in parallel spaced relation. An insulator 1050covers the electrodes and spacers 1052 are utilized to separate the rearpanel 1044 from the front panel 1042.

The front panel 1042 includes a glass face plate 1054 which istransparent when observed by the viewer (V). Printed onto the rearsurface of the glass face plate 1054 are a plurality of electrodes 1056(anodes) in parallel spaced relation. An insulator 1058 separates theelectrode from the pixels of phosphor powder 1060. The phosphor powder1060 is typically applied using a thick film paste. When the display1040 is assembled, the electrodes 1048 and 1056 are perpendicular toeach other, forming an XY grid. Thus, each pixel of phosphor powder canbe activated by the addressing an XY coordinate defined by theintersecting electrodes 1048 and 1056.

One of the problems currently encountered in plasma display devices isthe long decay time of the phosphor particles, which creates a “tail” ona moving image. Through control of the phosphor chemistry, suchdecay-related problems can be reduced. Further, the spherical,non-agglomerated nature of the phosphor particles improves theresolution of the plasma display panel.

Oxygen-containing phosphors according to the present invention which areparticularly useful for plasma displays include (Y,Gd)BO₃:Eu for red,Y₃Al₅O₁₂:Tb for blue/green, Zn₂SiO₄:Mn for green and BaMgAl O₁₄:Eu₂₃ forblue. The phosphors can advantageously be coated, such as with MgO, toreduce degradation from the plasma.

Another type of flat panel display is a field emission display (FED).These devices advantageously eliminate the size, weight and powerconsumption problems of CRT's while maintaining comparable imagequality, and therefore are particularly useful for portable electronics,such as for laptop computers. FED's generate electrons from millions ofcold microtip emitters with low power emission that are arranged in amatrix addressed array with several thousand emitters allocated to eachpixel in the display. The microtip emitters are located approximately0.2 millimeter from a cathodoluminescent phosphor screen which generatesthe display image. This allows for a thin, light-weight display.

FIG. 53 illustrates a high-magnification, schematic cross-section of anFED device according to an embodiment of the present invention. The FEDdevice 1080 includes a plurality of microtip emitters 1082 mounted on acathode 1084 which is attached to a backing plate 1086. The cathode isseparated from a gate or emitter grid 1088 by an insulating spacer 1090.Opposed to the cathode 1084 and separated by a vacuum is a faceplateassembly 1091 including phosphor pixel 1092 and a transparent anode1094. The phosphor pixel layers can be deposited using a paste orelectrophoretically. The FED can also include a transparent glasssubstrate 1096 onto which the anode 1094 is printed. During operation, apositive voltage is applied to the emitter grid 1088 creating a strongelectric field at the emitter tip 1082. The electrons 1098 migrate tothe faceplate 1091 which is maintained at a higher positive voltage. Thefaceplate collector bias is typically about 1000 volts. Several thousandmicrotip emitters 1082 can be utilized for each pixel in the display.

Oxygen-containing phosphors which are particularly useful for FEDdevices include Y₂O₃:Eu for red, ZnO for green and BAM:Eu for blue.These phosphors can be coated, such as with a metal oxide, since thehigh electron beam current densities can cause breakdown anddissociation of the phosphor host material. Dielectric coatings such asSiO₂ and Al₂O₃ can be used. Further, semiconducting coatings such asSnO₂ or In₂O₃ can be particularly advantageous to absorb secondaryelectrons.

Coatings for the oxygen-containing FED phosphors preferably have anaverage thickness of from about 1 to 10 nanometers, more preferably fromabout 1 to 5 nanometers. Coatings having a thickness in excess of about10 nanometers will decrease the brightness of the device since theelectron penetration depth of 1–2 kV electrons is only about 10nanometers. Such thin coatings can advantageously be monolayer coatings,as is discussed above.

The primary obstacle to further development of FED's is the lack ofadequate phosphor powders. FED's require low-voltage phosphor materials,that is, phosphors which emit sufficient light under low appliedvoltages, such as less than about 500 volts, and high current densities.The oxygen-containing phosphor powders of the present inventionadvantageously have improved brightness under such low applied voltagesand the coated phosphor particles resist degradation under high currentdensities. The improved brightness can be attributed to the highcrystallinity and high purity of the particles. Phosphor particles withlow crystallinity and high impurities due to processes such as millingdo not have the desirable high brightness. The phosphor particles of thepresent invention also have the ability to maintain the brightness andchromaticity over long periods of time, such as in excess of 10,000hours. Further, the spherical morphology of the phosphor powder improveslight scattering and therefore improves the visual properties of thedisplay. The small average size of the particles is advantageous sincethe electron penetration depth is only several nanometers, due to thelow applied voltage.

For each of the foregoing display devices, cathode ray tube devices andflat panel display devices including plasma display panels and fieldemission devices, it is important for the phosphor layer to be as thinand uniform as possible with a minimal number of voids. FIG. 54schematically illustrates a lay down of large agglomerated particles ina pixel utilizing conventional phosphor powders. The device 1100includes a transparent viewing screen 1102 and, in the case of an FED, atransparent electrode layer 1104. The phosphor particles 1106 aredispersed in pixels 1108. The phosphor particles are large andagglomerated and result in a number of voids and unevenness in thesurface. This results in decreased brightness and decreased imagequality.

FIG. 55 illustrates the same device fabricated utilizing powdersaccording to the present invention. The device 1110 includes transparentviewing screen 1112 and a transparent electrode 1114. The phosphorpowders 1116 are dispersed in pixels in 1118. The pixels are thinner andmore uniform than the conventional pixel. In a preferred embodiment, thephosphor layer constituting the pixel has an average thickness of notgreater than about 3 times the average particle size of the powder,preferably not greater than about 2 times the average particle size andeven more preferably not greater than about 1.5 times the averageparticle size. This unique characteristic is possible due to the uniquecombination of small particle size, narrow size distribution andspherical morphology of the phosphor particles. The device willtherefore produce an image having much higher resolution due to theability to form smaller, more uniform pixels and much higher brightnesssince light scattering is significantly reduced and the amount of lightlost due to non-luminescent particles is reduced.

Electroluminescent displays (EL displays) work by electroluminescence.EL displays are very thin structures which can have very small screensizes, such as few inches diagonally, while producing a very highresolution image. These displays, due to the very small size, areutilized in many military applications where size is a strictrequirement such as in aircraft cockpits, small hand-held displays andheads-up displays. These displays function by applying a high electricpotential between two addressing electrodes. EL displays are mostcommonly driven by an A.C. electrical signal. The electrodes are incontact with a semiconducting phosphor thin-film and the large potentialdifference creates hot electrons which move through the phosphor,allowing for excitation followed by light emission.

An EL display is schematically illustrated in FIGS. 56 and 57. The ELdisplay device 1120 includes a phosphor layer 1122 sandwiched betweentwo dielectric insulating layers 1124 and 1126. On the back side of theinsulating layers is a backplate 1128 which includes row electrodes1130. On the front of the device is a glass faceplate 1132 whichincludes transparent column electrodes 1134, such as electrodes madefrom transparent indium tin oxide.

While current electroluminescent display configurations utilize a thinfilm phosphor layer 1122 and do not typically utilize phosphor powders,the use of very small monodispersed phosphor particles according to thepresent invention is advantageous for use in such devices. For example,small monodispersed particles could be deposited on a glass substrateusing a thick film paste and sintered to produce a well connected filmand therefore could replace the expensive and material-limited CVDtechnology currently used to deposit such films. Such a well-connectedfilm could not be formed from large, agglomerated phosphor particles.Similarly, composite phosphor particles are a viable alternative to therelatively expensive multilayer stack currently employed inelectroluminescent displays. Thus, a composite phosphor particlecomprising the phosphor and a dielectric material could be used.

Another display device for which the phosphors according to the presentinvention are useful are liquid crystal displays (LCD), and inparticular active matrix liquid crystal displays (AMLCD). Such LCDdisplays are currently used for a majority of laptop computer displayscreens. The key element of an LCD device is the liquid crystal materialwhich can be influenced by an electric field to either transmit light orblock light.

LCD displays work by producing a light field and filtering light fromthe field using the liquid crystal material to produce an image. As aresult, only about 3% of the light emitted by the underlying phosphorscreen is transmitted to the viewer. Therefore, the phosphors accordingto the present invention having a higher brightness can provide LCDdisplays having increased brightness and contrast.

Another use for phosphor powders according to the present invention isin the area of electroluminescent lamps. Electroluminescent lamps areformed on a rigid or flexible substrate, such as a polymer substrate,and are commonly used as back lights for membrane switches, cellularphones, watches, personal digital assistants and the like. A simpleelectroluminescent lamp is schematically illustrated in FIG. 58. Thedevice 1140 includes a phosphor powder/polymer composite 1142 issandwiched between two electrodes 1144 and 1146, the front electrode1144 being transparent. The composite layer 1142 includes phosphorparticles 1148 dispersed in a polymer matrix 1150.

Electroluminescent lamps can also be formed on rigid substrates, such asstainless steel, for use in highway signage and similar devices. Therigid device includes a phosphor particle layer, a ceramic dielectriclayer and a transparent conducting electrode layer. Such devices aresometimes referred to as solid state ceramic electroluminescent lamps(SSCEL). To form such rigid devices, a phosphor powder is typicallysprayed onto a rigid substrate.

Additional colors, higher reliability and higher brightness powders arecritical needs for the electroluminescent lamp industry to supplydesigners with the ability to penetrate new market segments. Thephosphor layers should also be thinner and denser, without sacrificingbrightness, to minimize water intrusion and eliminate light scattering.Higher brightness electroluminescent lamps require thinner phosphorlayers, which requires smaller particle size phosphor powders thatcannot produced by conventional methods. Such thinner layers will alsouse less phosphor powder. Presently available EL lamps utilize powdershaving an average size of about 5 μm or higher, typically much higher.The phosphor powders of the present invention having a small particlesize and a narrow size distribution will enable the production ofbrighter and more reliable EL lamps that have an increasedlife-expectancy Further, the phosphor powders of the present inventionwill enable the production of EL lamps wherein the phosphor layer has asignificantly reduced thicknes, without sacraficing brightness or otherdesirable properties. Conventional EL lamps have phosphor layers on theorder of 100 μm thick. The powders of the present inventionadvantageously enable the production of an EL lamp having a phosphorlayer that is not greater than about 15 μm thick, such as not greaterthan about 10 μm thick. The phosphor layer is preferably not thickerthan about 3 times the weight average particle size, more preferably notgreater than about 2 times the weight average particle size.

As stated above, electroluminescent lamps are becoming increasinglyimportant for back lighting alphanumeric displays in small electronicdevices such as cellular phones, pagers, personal digital assistance,wrist watches, calculators and the like. They are also useful inapplications such as instrument panels, portable advertising displays,safety lighting, emergency lighting for rescue and safety devices,photographic backlighting, membrane switches and other similarapplications. One of the problems associated with electroluminescentdevices is that they generally require the application of alternatingcurrent (AC) voltage to produce light. A significant obstacle to thedevelopment of the useful direct current electroluminescent (DCEL)devices is a need for a phosphor powder that will function adequatelyunder a DC electric field. The phosphor powder for functioning under aDC electric field should meet at least three requirements: 1) theparticles should have a small average particle size; 2) the particlesshould have a uniform size, that is, the particle should have a narrowsize distribution with no large particles or agglomerates; and 3) theparticles should have good luminescence properties, particularly a highbrightness. The phosphor powders of the present invention advantageouslymeet these requirements. Therefore, the phosphor powders of the presentinvention will advantageously permit the use of electroluminescentdevices without requiring an inverter to convert a DC voltage to an ACvoltage. Such devices are not commercially available at this time. Whenutilized in a device applying DC voltage, it is preferred to coat thephosphor particles with a thin layer of a conductive metal, such ascopper, or a conductive compound such as copper sulfide.

The oxygen-containing phosphors of the present invention are also usefulas taggents for security purposes. In this application the phosphors,typically photoluminescent phosphors, which are undetectable undernormal lighting, become visible upon illumination by a particularenergy, typically infrared radiation, emitting characteristicwavelengths, typically in the ultraviolet spectrum.

For security purposes, the phosphor particles are dispersed into aliquid vehicle which can be applied onto a surface by standard inkdeposition methods, such as by using an ink jet or a syringe, or byscreen printing. The phosphor particles of the present invention, havinga small size and narrow size distribution, will permit better controlover the printed feature size and complexity. The methodology of thepresent invention also permits unique combinations of phosphor compoundsthat are not available using conventional methods. Such taggents can beapplied to currency, secure documentation, explosives and munitions, orany other item that may require coding. The phosphor powders canadvantageously be dispersed in an ink which is then used to form indiciaon a document or other item, such as a postal envelope.

Useful phosphor compounds for taggent applications include Y₂O₃:RE¹,RE², wherein RE^(1,2) are rare-earth elements, such as Eu and Yb. Suchphosphors emit visible light upon excitation by an infrared source. Thephosphor powders of the present invention provide many advantages insuch applications. For example, the small, monodispersed nature of theparticles makes the particles easy to supply in smaller quantities.

Up-convertor phosphors are also useful in immunoassay applicationsImmunoassays are bioactive agent detectors designed to detect chemicalsin the bloodstream, such as sugars, insulin or narcotics. The phosphoris delivered to the biological substrate and the interaction between thesubstrate and the underlying phosphor results in a detected color shiftwhich can be correlated with the concentration of the initial bioactivemolecule present in the sample. For example, incident infrared light canresult in a detectable ultraviolet signal from the phosphor. Theup-convertor phosphors of the present invention used for suchimmunoassay applications preferably have an average particle size offrom about 0.1 μm to about 0.4 μm and are preferably coated to bind thebiologically active molecule. The particles are frequently coated, suchas with SiO₂, to enhance to binding of the phosphor to the biologicalsubstrate and for biocompatibility.

Another application of the phosphor powders of the present invention isin the field of x-ray imaging, such as for medical diagnostics. X-rayphosphors are utilized in x-ray intensifying screens which enhance thephotographic image formed on photographic film while reducing the x-raydose on the object during medical radiographic procedures. Preferredphosphor compounds for use in these screens are colorless single phasematerials with high crystallinity. It is preferred that the particleshave a small average particle size so that they can form a thin layerwhen applied to the screen, while maintaining a void-free uniform layer.Coating agents can be utilized create conditions such that all the lightemitted from the phosphor is emitted with minimum loss due toscattering, self adsorption and the like. Specific oxygen-containingphosphor compounds useful for x-ray imaging include CaWO₄, LaOBr:Tm,Gd₂O₂S:Tb and YTaO₄.

In addition to the foregoing, the oxygen-containing phosphors of thepresent invention can also be used as target materials for thedeposition of phosphor thin-films by electron beam deposition,sputtering and the like. The particles can be consolidated to form thetarget for the process. The homogenous concentration of activator ionsin the particles will lead to more uniform and brighter film. Thephosphor powders can also be used to adjust the color of light emittingdiodes.

For many of the foregoing applications, phosphor powders are oftendispersed within a paste which is then applied to a surface to obtain aphosphorescent layer. These pastes are commonly used forelectroluminescent lamps, FED's, plasma displays, CRT's, lamp phosphorsand thick-film electroluminescent displays. The powders of the presentinvention offer many advantages when dispersed in such a paste. Forexample, the powders will disperse better than non-spherical powders ofwide size distribution and can therefore produce thinner and moreuniform layers with a reduced lump count. Such a thick film paste willproduce a brighter display. The packing density of the phosphors willalso be higher. The number of processing steps can also beadvantageously reduced. For example, in the preparation ofelectroluminescent lamps, two dielectric layers are often needed tocover the phosphor paste layer because many of the phosphor particleswill be large enough to protrude through one layer. Spherical particlesthat are substantially uniform in size will eliminate this problem andthe EL lamp will advantageously require one dielectric layer.

One preferred class of intermediate products according to the presentinvention are thick film paste compositions, also referred to as thickfilm inks. These pastes are particularly useful for the application ofthe phosphor particles onto a substrate, such as for use in a flat paneldisplay, as is discussed more fully hereinbelow.

In the thick film process, a viscous paste that includes a functionalparticulate phase, such as phosphor powder, is screen printed onto asubstrate. A porous screen fabricated from stainless steel, polyester,nylon or similar inert material is stretched and attached to a rigidframe. A predetermined pattern is formed on the screen corresponding tothe pattern to be printed. For example, a UV sensitive emulsion can beapplied to the screen and exposed through a positive or negative imageof the design pattern. The screen is then developed to remove portionsof the emulsion in the pattern regions.

The screen is then affixed to a printing device and the thick film pasteis deposited on top of the screen. The substrate to be printed is thenpositioned beneath the screen and the paste is forced through the screenand onto the substrate by a squeegee that traverses the screen. Thus, apattern of traces and/or pads of the paste material is transferred tothe substrate. The substrate with the paste applied in a predeterminedpattern is then subjected to a drying and heating treatment to adherethe functional phase to the substrate. For increased line definition,the applied paste can be further treated, such as through aphotolithographic process, to develop and remove unwanted material fromthe substrate.

Thick film pastes have a complex chemistry and generally include afunctional phase, a binder phase and an organic vehicle phase. Thefunctional phase can include the phosphor powders of the presentinvention which provide a luminescent layer on a substrate. The particlesize, size distribution, surface chemistry and particle shape of theparticles all influence the rheology of the paste.

The binder phase is typically a mixture of inorganic binders such asmetal oxide or glass frit powders. For example, PbO based glasses arecommonly used as binders. The function of the binder phase is to controlthe sintering of the film and assist the adhesion of the functionalphase to the substrate and/or assist in the sintering of the functionalphase. Reactive compounds can also be included in the paste to promoteadherence of the functional phase to the substrate.

Thick film pastes also include an organic vehicle phase that is amixture of solvents, polymers, resins or other organics whose primaryfunction is to provide the appropriate rheology (flow properties) to thepaste. The liquid solvent assists in mixing of the components into ahomogenous paste and substantially evaporates upon application of thepaste to the substrate. Usually the solvent is a volatile liquid such asmethanol, ethanol, terpineol, butyl carbitol, butyl carbitol acetate,aliphatic alcohols, esters, acetone and the like. The other organicvehicle components can include thickeners (sometimes referred to asorganic binders), stabilizing agents, surfactants, wetting agents andthe like. Thickeners provide sufficient viscosity to the paste and alsoacts as a binding agent in the unfired state. Examples of thickenersinclude ethyl cellulose, polyvinyl acetate, resins such as acrylicresin, cellulose resin, polyester, polyamide and the like. Thestabilizing agents reduce oxidation and degradation, stabilize theviscosity or buffer the pH of the paste. For example, triethanolamine isa common stabilizer. Wetting agents and surfactants are well known inthe thick film paste art and can include triethanolamine and phosphateesters.

The different components of the thick film paste are mixed in thedesired proportions in order to produce a substantially homogenous blendwherein the functional phase is well dispersed throughout the paste. Thepowder is often dispersed in the paste and then repeatedly passedthrough a roll-mill to mix the paste. The roll mill can advantageouslybreak-up soft agglomerates of powders in the paste. Typically, the thickfilm paste will include from about 5 to about 95 weight percent, such asfrom about 60 to 80 weight percent, of the functional phase, includingthe phosphor powders of the present invention.

Phosphor paste compositions are disclosed in U.S. Pat. No. 4,724,161,U.S. Pat. No. 4,806,389, U.S. Pat. No. 4,902,567 which are incorporatedherein by reference in their entirety. Generally, phosphors aredeaggregated and are combined with organic additives to form the paste.

Some applications of thick film pastes, such as for forminghigh-resolution display panels, require higher tolerances than can beachieved using standard thick-film technology, as is described above. Asa result, some thick film pastes have photo-imaging capability to enablethe formation of lines and traces with decreased width and pitch. Inthis type of process, a photoactive thick film paste is applied to asubstrate substantially as is described above. The paste can include,for example, a liquid vehicle such as polyvinyl alcohol, that is notcross-linked. The paste is then dried and exposed to ultraviolet lightthrough a photomask to polymerize the exposed portions of paste and thepaste is developed to remove unwanted portions of the paste. Thistechnology permits higher density lines and pixels to be formed. Thecombination of the foregoing technology with the phosphor powders of thepresent invention permits the fabrication of devices with resolution andtolerances as compared to conventional technologies using conventionalphosphor powders.

EXAMPLES

Y₂O₃ Phosphors

To demonstrate the advantages of the present invention, europium dopedyttria phosphors (Y₂O₃:Eu³⁺) were prepared under a variety ofconditions. This phosphor compound is a red phosphor with a peakexcitation wavelength at 253 nanometers. This phosphor compound is oneof the most widely used red phosphors and is useful in manyapplications.

For each of these examples, the powders were produced in accordance withthe teachings of the present invention. An aerosol of a precursorsolution was generated using an ultrasonic atomization technique. Theultrasonic transducers had a frequency of about 1.6 MHZ. The dropletswere carried through a tubular furnace in a carrier gas (air) withoutclassifying the droplets with an impactor. The average residence time ofthe particles in the furnace was estimated to be about 10 seconds. Theprecursors were yttrium nitrate (Y(NO₃)₃.6H₂O and europium nitrate(Eu(NO₃)₃.6H₂O). Except as noted, the concentration of the precursorsfor all experiments yielded 2.9 weight percent yttria in the solution,or 10 grams yttrium nitrate per 100 ml of water.

A first set of experiments was conducted to determine the optimumreaction temperature for producing Y₂O₃:Eu with 1 atomic percenteuropium. The reaction temperature was varied at 100° C. intervalsbetween 700° C. and 1500° C. The relative photoluminescent intensityincreased from 700° C. to 1000° C. and peaked at a reaction temperatureof 1000° C. From 1000° C. to 1500° C., the relative intensity steadilydecreased. Also, the average crystallite size of the powders steadilyincreased with reaction temperature from 700° C. to 1500° C., andincreased from about 15 nanometers (700° C.) to about 50 nanometers(1500° C.). The average crystallite size at 1000° C. was about 28nanometers.

Based on the foregoing, further examples were prepared at a reactiontemperature of 1000° C. The europium concentration was varied from about1 atomic percent to about 15 atomic percent. The relativephotoluminescent intensity of the powders increased steadily to about 11atomic percent Eu, and then rapidly decreased. The maximum intensityoccurred at an Eu level of about 11 atomic percent. It is believed thatphosphor powders produced according to the present invention canadvantageously incorporate this increased amount of activator ion due tothe improved atomic mixing of the activator ion in the host lattice,permitting higher amounts of activator ion to be utilized duringluminescence.

To determine the effect of annealing on the Y₂O₃:Eu powders of thepresent invention, powders incorporating 2 atomic percent Eu that wereproduced at 1000° C. were annealed under varying conditions to determinethe effect of the annealing temperature. The powder was placed in aquartz boat and heated at a rate of 10° C. per minute and allowed todwell at a maximum temperature for about 6 minutes in stagnant air. Theannealing temperature was varied from 1000° C. to 1600° C. The highestrelative photoluminescent intensity was observed at an annealingtemperature of 1500° C. At annealing temperatures below about 1400° C.,the brightness of the powder changed little. The average crystallitesize increased steadily from about 22 nanometers to about 68 nanometersat 1500° C. It was also observed that when the brightest non-annealedpowder (produced at 1000° C. with 10 atomic percent Eu) was annealed at1400° C. for about 6 minutes, the photoluminescent intensity increasedby about 55%.

In a further set of Examples, urea was added to the precursor solutionto increase the density of the Y₂O₃:Eu phosphor. Specifically, urea inamounts ranging from 0.5 mole equivalents to about 4 mole equivalentswere added to the precursor solution. The bulk density was measuredusing a standard helium pychnometry technique. When produced withouturea, the particles had a density of about 4.1 g/cc, which is slightlygreater than 80 percent of the theoretical density (5.01 g/cc). Theaddition of 0.5 mole equivalents of urea increased the density to about5.0 g/cc or 99% of the theoretical density. One mole equivalent of ureayielded a powder having slightly decreased density, about 4.95. Higheradditions of urea produced powder having a steadily decreasing density.

Further experiments were conducted to determine the optimumconcentration of the nitrate precursor to yield the highest productionrate. The solubility of the nitrate salt in a water solution is about57.3 weight percent which corresponds to about 28 weight percent yttria.Solutions were prepared that incorporated 5, 10 and 20 weight percentY₂O₃. The highest production rate occurred with the 5 weight percentsolution. Higher concentrations of the precursor appeared to produceexploded particles with many fragments and debris. The best morphologywas produced at 5 weight percent yttria since the powder had minimaldebris. An example of a Y₂O₃:Eu phosphor powder produced according tothe present invention is illustrated in FIG. 59. The powder was producedfrom yttrium nitrate and europium nitrate, with urea added to theprecursor solution. The solution was atomized using an ultrasonictransducer at a frequency of 1.6 MHz and the aerosol was passed throughan impactor. The reaction temperature was 1100° C. As is illustrated inFIG. 59, the powder had a small average particle size, narrow sizedistribution and a spherical morphology.

Using similar process parameters, other oxygen-containing phosphors wereproduced. For example, a ZnO:Zn intrinsic phosphor powder was producedin a similar fashion from a zinc nitrate precursor at a reactortemperature of from about 700° C. to about 900° C. The degree to whichthe ZnO is reduced, and hence the luminescence characteristics, canadvantageously be controlled by varying the carrier gas composition. AnSEM photomicrograph of the ZnO powder is illustrated in FIG. 60.

Spray Conversion to Form Y₃Al₅O₁₂:Ce

Y₃Al₅O₁₂:Ce, known as yttria-alumina garnet (YAG), was formed by aspray-conversion process according to the present invention. YAG is acomplicated ternary oxide that cannot be readily formed by ordinaryspray pyrolysis routes.

Accordingly, 1 gram of yttrium nitrate and 1.63 grams of aluminumnitrate (3/5 ratio) was dissolved in distilled water with severalmilligrams of cerium nitrate. The solution was nebulized using anultrasonic transducer at a frequency of about 1.6 MHZ and was carried inair to a tubular furnace heated to a peak reaction temperature. In afirst experiment, the peak reaction temperature was about 1100° C. Theresulting powder was composed of YAlO₃ and amorphous Al₂O₃. Furtherannealing of the powder in air at 1000° C. did not change thecomposition.

In a second experiment, the same precursor solution was nebulized andcarried in air through a tubular furnace heated to a temperature ofabout 125° C. The resulting white powder had a small average particlesize and other properties commensurate with the present invention. Thisintermediate product was then heated to 1000° C. in air for 2 hours. Theresulting powder was highly crystalline Y₃Al₅O₁₂:Ce having a smallaverage particle size.

While various embodiments of the present invention have been describedin detail, it is apparent that modifications and adaptations of thoseembodiments will occur to those skilled in the art. However, it is to beexpressly understood that such modifications and adaptations are withinthe spirit and scope of the present invention.

1. A display device for conveying visual graphics and information,comprising: a) a plurality of pixel regions comprising phosphor powderlayers; and b) an excitation source adapted to stimulate said phosphorpowder to emit light to be viewed by a viewer; wherein said phosphorpowder comprises oxygen-containing phosphor particles selected from thegroup consisting of borate particles and aluminate particles that aresubstantially spherical and have a weight average particle size of notgreater than about 5 μm.
 2. A display device as recited in claim 1,wherein said weight average particle size is from about 0.3 μm to about3 μm.
 3. A display device as recited in claim 1, wherein said powder hasa particle size distribution wherein at least about 90 weight percent ofsaid particles are not larger than twice said average particle size. 4.A display device as recited in claim 1, wherein said phosphor powderlayers have an average thickness of not greater than about 3 times saidaverage particle size.
 5. A display device as recited in claim 1,wherein said phosphor powder layers have an average thickness of notgreater than about 2 times said average particle size.
 6. A displaydevice as recited in claim 1, wherein said phosphor particles comprise ayttrium-gadolinium borate host material.
 7. A display device as recitedin claim 1, wherein said phosphor particles comprise a host materialselected from the consisting of Y₃Al₅O₁₂ and BaMgAl₁₄O₂₃.
 8. A flatpanel display, comprising: a) an excitation source adapted to stimulatea phosphor; and b) a viewing panel proximate to said excitation source,comprising a transparent substrate having disposed thereon anoxygen-containing phosphor powder defining pixels, wherein said phosphorpowder comprises substantially spherical aluminate particles having aweight average particle size of not greater than about 10 μm.
 9. A flatpanel display as recited in claim 8, wherein said weight averageparticle size is not greater than about 5 μm.
 10. A flat panel displayas recited in claim 8, wherein said weight average particle size is fromabout 0.3 μm to about 3 μm.
 11. A flat panel display as recited in claim8, wherein said powder has a particle size distribution wherein at leastabout 90 weight percent of said particles are not larger than twice saidaverage particle size.
 12. A flat panel display as recited in claim 8,wherein said pixels comprise a phosphor powder layer having an averagethickness of not greater than about 3 times said average particle size.13. A flat panel display as recited in claim 8, wherein said pixelscomprise a phosphor powder layer having an average thickness of notgreater than about 2 times said average particle size.
 14. A flat paneldisplay as recited in claim 8, wherein said flat panel display is afield emission display.
 15. A flat panel display as recited in claim 8,wherein said flat panel display is a plasma display.
 16. A flat paneldisplay as recited in claim 8, wherein said aluminate phosphor powdercomprises a host material selected from the group consisting of Y₃Al₅O₁₂and BaMgAl₁₄O₂₃.
 17. A display device as recited in claim 8, whereinsaid phosphor particles comprise a host material consisting essentiallyof BaMgAl₁₄O₂₃.
 18. A field emission display, comprising: (a) a backplate portion comprising a plurality of electron tip emitters; (b) atransparent front plate portion comprising a layer of phosphor powdercomprising oxygen-containing, substantially spherical aluminate phosphorparticles, wherein said phosphor particles have a weight averageparticle size of not greater than about 5 μm.
 19. A field emissiondisplay as recited in claim 18, wherein said phosphor particles compriseY₃Al₅O₁₂ as a host material.
 20. A field emission display as recited inclaim 18, wherein said phosphor particles comprise BaMgAl₁₄O₂₃ as a hostmaterial.
 21. A field emission display as recited in claim 18, whereinsaid phosphor particles comprise crystallites having an averagecrystallite size of at least about 25 nanometers.
 22. A field emissiondisplay as recited in claim 18, wherein said average particle size isfrom about 0.3 μm to about 3 μm.
 23. A field emission display as recitedin claim 18, wherein said phosphor particles are coated phosphorparticles.
 24. A field emission display as recited in claim 18, whereinsaid phosphor particles are coated phosphor particles comprising a metaloxide coating having a thickness of from about 1 to about 10 nanometers.25. A field emission display as recited in claim 18, wherein saidphosphor particles are coated phosphor particles comprising a metaloxide coating selected from the group consisting of Al₂O₃, SiO₂, ZnO,SnO₂ and In₂O₃.
 26. A field emission display as recited in claim 18,wherein said phosphor particles form a pixel layer having an averagethickness of not greater than about 3 times said average particle size.27. A method for securing a document, comprising the step of applying aphosphor powder to said document, wherein said phosphor powder comprisesoxygen-containing phosphor particles, wherein said phosphor particlesare substantially spherical and have a weight average particle size offrom about 0.3 μm to about 3 μm and wherein at least about 90 weightpercent of said particles are not larger than twice said averageparticle size.
 28. A method for securing a document as recited in claim27, wherein said phosphor particles comprise crystallites having anaverage crystallite size of at least about 25 nanometers.
 29. A methodfor securing a document as recited in claim 27, wherein said document iscurrency.
 30. A plasma display panel, comprising: a) a rear panelcomprising a plurality of row electrodes; b) a front panel comprising aplurality of column electrodes, wherein said row electrodes and saidcolumn electrodes are in perpendicular relation to form a plurality ofaddressable x-y coordinates; c) a phosphor powder dispersed on asubstrate disposed between said electrodes, wherein said phosphor powdercomprises a host material selected from the group consisting of ZnO andaluminates, and comprises particles having a weight average particlesize of not greater than about 5 μm and a particle size distributionwherein at least about 90 weight percent of said particles are notlarger than twice said average particle size.
 31. A plasma display panelas recited in claim 30, wherein said particles are substantiallyspherical.
 32. A plasma display panel as recited in claim 30, whereinsaid average particle size is from about 0.3 μm to about 3 μm.
 33. Aplasma display panel as recited in claim 30, wherein said phosphorparticles comprise crystallites having an average crystallite size of atleast about 25 nanometers.
 34. A plasma display panel as recited inclaim 30, wherein said phosphor powder is dispersed in a layer having anaverage thickness of not greater than about three times said averageparticle size.
 35. A plasma display panel as recited in claim 30,wherein said phosphor particles comprise a metal oxide host materialselected from the group consisting of Y₃Al₅O₁₂, ZnO and BaMgAl₁₄O₂₃. 36.A method as recited in claim 27, wherein said phosphor particlescomprise Y₂O₃:RE¹, RE², where RE¹ and RE² are selected from the rareearth elements.